This page in your language

فارسی

العربية

Deutsch

Español

Français

日本語

한국어

Português

中文

हिन्दी

Home
Learn
Japanese
Vocabulary
同意する

B1 verb #2,000 most common 10 min read

同意する

doui suru Top 2,000

At the A1 level, you don't need to use '同意する' (dōi suru) very often. Instead, you usually say 'はい' (yes) or 'いいですよ' (it's okay). However, you might see this word on websites or in simple forms. It means 'to say yes' to a rule. For example, when you see a checkbox that says '同意する', it just means you agree to the rules of the website. You should learn it as a 'sight word' first. In simple conversation, if someone asks 'いいですか？' (Is it okay?), you can just say 'はい' instead of '同意します'. '同意する' is very formal, like a businessman or a lawyer talking. As an A1 learner, focusing on '賛成' (sansei) for 'I like that idea' is more useful. But remember: if you see '同意' on a button, click it to move to the next page!

At the A2 level, you are starting to handle more formal situations, like visiting a doctor or signing up for a gym. You will encounter '同意' (dōi) in these places. It is important to know that '同意する' means 'to give consent.' For example, 'この紙に同意のサインをしてください' (Please sign your consent on this paper). You might also hear it in news clips. You should start to understand that '同意する' is used with the particle 'に' (ni). So, you say '意見に同意する' (agree to an opinion). At this level, you can stick to '私もそう思います' (I think so too) for daily talk, but use '同意します' when you want to sound very serious or professional. It shows you are moving beyond basic Japanese and understanding formal structures.

At the B1 level, '同意する' becomes a key verb for participating in discussions and expressing your stance clearly. You should be able to use it to agree with someone's proposal in a meeting or a classroom debate. The nuance here is 'intellectual agreement.' You aren't just saying 'I like it'; you are saying 'I have considered your point and I concur.' You will also learn to use it in the negative: '同意できません' (I cannot agree). This is a polite but firm way to disagree in a professional setting. You should also be comfortable with the noun form '同意' and common phrases like '同意を得る' (to get consent). Understanding the difference between '同意' (formal agreement) and '賛成' (supporting an idea) is a hallmark of the B1 level.

At the B2 level, you should use '同意する' with precision in complex social and professional contexts. You will encounter it in legal texts, contracts, and high-level business negotiations. You should understand the passive form '同意を得られる' (to be able to get consent) and the potential for nuance using adverbs like '全面的に' (completely) or '条件付きで' (conditionally). At this level, you are expected to know that '同意' implies a formal alignment of will. You might use it in an essay to discuss social consensus: '国民の同意を得る必要がある' (It is necessary to get the consent of the citizens). You should also be aware of related terms like '承諾' (acceptance) and '合意' (mutual agreement/settlement) and know when '同意' is the most appropriate choice.

At the C1 level, you must master the subtle legal and philosophical implications of '同意する'. This includes understanding 'informed consent' (説明と同意) in medical ethics and the nuances of 'implied consent' in legal frameworks. You should be able to use the word in academic writing to discuss the alignment of different theories or the consensus within a scientific community. You will also use it in high-level diplomatic or corporate contexts where '同意' might be a step toward a '合意' (settlement). Your usage should reflect an understanding of the power dynamics involved—who is seeking consent and who is granting it. You should also be able to critique the use of the word in political rhetoric, identifying when '同意' is being manufactured or coerced.

At the C2 level, your command of '同意する' is indistinguishable from a highly educated native speaker. You understand its use in classical-style legal language and its role in the history of Japanese civil law. You can navigate the most complex linguistic environments, such as interpreting for international treaties where the distinction between 'agreement,' 'concurrence,' and 'consent' must be perfectly reflected in Japanese. You are aware of the deep cultural history of consensus-building (nemawashi) and how '同意する' serves as the final, formal step in that process. You can use the word with absolute precision, choosing it over '合意', '承諾', or '妥協' (compromise) to convey the exact legal and psychological state of the parties involved.

Practice it Examples Notes & Tips

同意する in 30 Seconds

同意する (dōi suru) is a formal verb meaning 'to agree' or 'to consent,' commonly used in professional, legal, and administrative contexts in Japan.
It is a Suru-verb that typically takes the particle 'に' (ni) to mark the proposal, opinion, or person that is being agreed with.
Unlike casual agreement, it implies a formal alignment of will and is the standard term for 'Terms of Service' or medical consent.
Commonly paired with adverbs like '全面的に' (completely) or used in the noun form '同意' (consent) in phrases like '同意を得る'.

The Japanese verb 同意する (どういする - dōi suru) is a formal and precise term used to express agreement, consent, or concurrence. At its linguistic core, the word is composed of two kanji: 同 (dō), meaning 'same' or 'identical,' and 意 (i), meaning 'mind,' 'intention,' or 'thought.' When combined with the light verb suru (to do), it literally translates to 'doing the same mind' or 'aligning intentions.' This etymological foundation highlights that the word is not just about saying 'yes,' but about a formal alignment of perspectives or legal acceptance of terms.

Formal Consent: In legal and administrative contexts, this term is the standard way to indicate that one has read and accepted terms, such as a 'Terms of Service' agreement (利用規約に同意する).

Intellectual Concurrence: In academic or professional debates, it signifies that one finds the logic of another's argument sound and chooses to support it formally.

彼は私の提案に全面的に同意すると言った。
(He said he completely agrees with my proposal.)

Understanding the nuance of dōi suru requires distinguishing it from more casual forms of agreement. While sansei suru (賛成する) implies being 'in favor' of something or supporting a cause, dōi suru is more about the act of reaching a shared conclusion or giving permission. It is the difference between 'I support your idea' and 'I formally agree with your statement.' This distinction is crucial in B1-level communication where precision in professional and social interactions begins to matter significantly.

プライバシーポリシーに同意する必要があります。
(You need to agree to the privacy policy.)

Furthermore, the word is often used in the negative form dōi dekinai (cannot agree) to express a formal disagreement without being overly aggressive. It maintains a level of professional distance. In Japanese culture, where harmony (wa) is valued, using a formal word like dōi can sometimes soften the blow of a disagreement by making it sound like a difference in formal position rather than a personal conflict.

その意見には同意することができません。
(I cannot agree with that opinion.)

Social Nuance: In social settings, using '同意する' instead of '賛成' or 'いいよ' (okay) signals that the topic is serious, such as a marriage proposal or a major financial decision.

両親は私たちの結婚に同意するはずだ。
(My parents should consent to our marriage.)

条件に同意するなら、ここにサインしてください。
(If you agree to the conditions, please sign here.)

To summarize, 同意する is a versatile but formal verb that bridges the gap between personal opinion and legal consent. It is an essential word for anyone navigating professional environments or formal documentation in Japan, ensuring that your 'yes' is understood with the appropriate level of gravity and commitment.

Using 同意する correctly involves understanding its grammatical structure and the specific particles that accompany it. The most common particle used with this verb is に (ni), which marks the object of agreement—the proposal, the opinion, or the person. Unlike English where we say 'agree with,' in Japanese, you 'agree to' (に) the target.

Basic Grammar Structure: [Target] + に + 同意する. Example: 案に同意する (Agree to the plan).

私はあなたの意見に同意する。
(I agree with your opinion.)

In formal writing or business speech, you will often see it used in the passive form 同意される (to be agreed upon) or the potential form 同意できる (can agree). When asking for agreement, the phrase ご同意いただけますか？ (Could I have your agreement?) is a highly polite way to seek consensus in a corporate setting.

Adverbial Modifiers: Commonly paired with adverbs like 全面的に (zenmenteki ni - completely), 強く (tsuyoku - strongly), or 渋々 (shibushibu - reluctantly).

彼は渋々その条件に同意することにした。
(He decided to reluctantly agree to those conditions.)

One of the most frequent uses of 同意する is in the noun form 同意 (dōi). You will see phrases like 同意を得る (dōi o eru - to obtain consent) or 同意を求める (dōi o motomeru - to seek agreement). In medical contexts, インフォームド・コンセント (informed consent) is often translated as 説明と同意 (explanation and consent).

手術の前に家族の同意が必要です。
(Family consent is required before the surgery.)

Negative Usage: To say 'I don't agree,' you can say 同意しません (formal) or 同意できない (I cannot agree). The latter is often preferred as it sounds less like a personal rejection and more like a logical impossibility.

その点については同意するわけにはいきません。
(I simply cannot agree on that point.)

全員がその決断に同意するまで話し合いを続けた。
(They continued the discussion until everyone agreed to the decision.)

When writing, remember that 同意する is a Suru-verb. This means it follows all standard Suru-verb conjugations (同意した, 同意しない, 同意して, etc.). Mastering its use with the particle 'に' and understanding its noun-form applications will allow you to navigate complex Japanese social and professional structures with confidence.

The verb 同意する is ubiquitous in specific domains of Japanese life, primarily those involving formality, legality, and collective decision-making. You are unlikely to hear it in a casual conversation between friends about where to eat lunch, but you will encounter it daily in professional and digital environments.

Digital Environment: Every time you sign up for a Japanese app or service, you will see '利用規約に同意する' (Agree to the Terms of Service). It is the standard button text for confirmation.

「同意して次へ」のボタンをクリックしてください。
(Please click the 'Agree and Proceed' button.)

In the workplace, 同意する is used during meetings, especially when a consensus is being reached. A manager might ask, 'この方針に同意していただけますか？' (Can you agree to this policy?). It is also common in written reports and emails when confirming that a client or partner has accepted a proposal.

News and Media: News broadcasts frequently use the term when reporting on international treaties, political alliances, or legal settlements. '両国は新条約に同意した' (Both countries agreed to the new treaty).

政府は与党の案に同意する方針を固めた。
(The government has firmed up its policy to agree to the ruling party's proposal.)

Another common setting is in academic or intellectual discourse. When a scholar agrees with another's theory, they might say, 'その説に同意します' (I agree with that theory). It sounds more rigorous and considered than '賛成' (sansei), which can sometimes sound like mere personal preference.

多くの専門家が彼の意見に同意するだろう。
(Many experts will likely agree with his opinion.)

Legal and Formal Documents: Contracts often include clauses like '甲は乙の提案に同意するものとする' (Party A shall agree to Party B's proposal).

書面での同意が必要です。
(Written consent is required.)

本人の同意なく個人情報を公開してはならない。
(Personal information must not be disclosed without the individual's consent.)

In summary, 同意する is the language of formal commitment. Whether you are navigating the web, working in a Japanese office, or reading the news, this word serves as a clear marker of consensus and legal or professional alignment.

While 同意する is a straightforward verb, learners often make subtle mistakes in its usage, particularly regarding its formality level, particle choice, and confusion with similar-sounding or similar-meaning words.

Mistake 1: Overusing in Casual Settings: Using '同意する' with friends when deciding on a movie or a restaurant sounds overly stiff and robotic. In these cases, '賛成' (sansei) or simply 'いいよ' (ii yo) is much more natural.

❌ 友達：「映画に行こう！」私：「同意する。」
✅ 友達：「映画に行こう！」私：「賛成！」
(Casual agreement should use 'Sansei'.)

Another common error is the incorrect use of particles. Many learners, influenced by the English 'agree with,' try to use the particle to (と) when ni (に) is the standard choice for the object of agreement. While to is possible in specific 'mutual' contexts, ni is the safe and correct default for 'agreeing to' a proposal or opinion.

Mistake 2: Particle Confusion: Using 'と' (with) instead of 'に' (to/at) when referring to a plan or opinion. '案に同意する' is correct; '案と同意する' is usually incorrect.

❌ 彼の意見と同意する。
✅ 彼の意見に同意する。
(Always use 'ni' for the object of agreement.)

Confusion with 了解する (ryōkai suru) is also frequent. Ryōkai means 'understood' or 'roger,' often used in a military or hierarchical sense (boss to subordinate). It does not necessarily mean you agree with the logic, just that you have received the information. Dōi means you actually share the opinion.

Mistake 3: Dōi vs. Ryōkai: Using '同意' when you just mean 'I understand the instruction.' If your boss tells you to finish a report, you say '了解しました,' not '同意しました.'

❌ 上司：「明日までにやって。」私：「同意しました。」
✅ 上司：「明日までにやって。」私：「了解しました。」
(Use 'Ryōkai' for understanding tasks.)

❌ その案に同意をあげる。
✅ その案に同意を与える/示す。
(You don't 'give' (ageru) agreement like a gift; you 'show' (shimesu) or 'grant' (ataeru) it.)

Finally, remember that 同意する is an active choice. In Japanese, silence is often interpreted as agreement (the concept of 'unspoken consensus'), but using 同意する makes that agreement explicit and legally/professionally binding. Use it when you want to leave no room for doubt.

Japanese has a rich vocabulary for agreement, and choosing the right word depends on the context, the relationship between speakers, and the level of formality required. Here is how 同意する compares to its synonyms.

賛成する (Sansei suru): Meaning 'to be in favor of' or 'to support.' This is more common in voting or expressing a personal preference for a plan. While '同意' is about aligning minds, '賛成' is about taking a side.

私はその計画に賛成です。
(I am in favor of that plan.)

承諾する (Shōdaku suru): Meaning 'to accept' or 'to consent to a request.' This is specifically used when someone asks you for a favor or a contract, and you officially accept it. It is more transactional than '同意'.

依頼を承諾した。
(I accepted the request.)

納得する (Nattoku suru): Meaning 'to be convinced' or 'to understand and accept.' This focuses on the internal process of reaching agreement. You 'nattoku' when the explanation finally makes sense to you.

説明を聞いて納得した。
(I was convinced after hearing the explanation.)

同感 (Dōkan): Meaning 'same feeling.' This is a noun often used as '同感です' (I feel the same). It is less formal than '同意' and focuses on shared sentiment or empathy.

あなたの意見には全く同感です。
(I completely feel the same way about your opinion.)

肯定する (Kōtei suru): Meaning 'to affirm' or 'to say yes to.' This is the opposite of '否定する' (hitei suru - to deny). It is often used in logic or when confirming the truth of a statement.

彼はその事実を肯定した。
(He affirmed that fact.)

By understanding these differences, you can choose the word that best fits your intent. 同意する remains the most versatile choice for formal, intellectual, and legal agreement, making it a vital part of your B1-level Japanese toolkit.

How Formal Is It?

Formal

Difficulty Rating

Grammar to Know

Examples by Level

ここに同意のチェックをしてください。

Please check 'agree' here.

Noun '同意' used with particle 'の'.

ルールに同意しますか？

Do you agree to the rules?

Basic question form.

はい、同意します。

Yes, I agree.

Simple present tense.

同意しないなら、使えません。

If you don't agree, you can't use it.

Negative 'nai' form.

このボタンは「同意」です。

This button is 'Agree'.

Noun usage.

お父さんは同意しましたか？

Did your father agree?

Past tense 'shimashita'.

みんなで同意しましょう。

Let's all agree.

Volitional 'mashō' form.

同意が必要です。

Agreement is necessary.

Noun + ga hitsuyō desu.

彼の意見に同意しました。

I agreed with his opinion.

Using particle 'ni' for opinion.

この契約に同意できますか？

Can you agree to this contract?

Potential form 'dekimasu'.

母は私の留学に同意してくれた。

My mother agreed to my studying abroad.

Using 'te kureta' for a favor.

同意書にサインをしてください。

Please sign the consent form.

Compound noun '同意書' (consent form).

まだ同意していません。

I haven't agreed yet.

Present continuous negative.

だれも同意しなかった。

Nobody agreed.

Negative past tense.

プランに同意するなら、教えてください。

If you agree to the plan, please let me know.

Conditional 'nara'.

先生の考えに同意します。

I agree with the teacher's idea.

Formal agreement.

全面的にあなたの提案に同意します。

I completely agree with your proposal.

Using the adverb '全面的に' (completely).

両親の同意を得て、家を借りました。

I got my parents' consent and rented a house.

Phrase '同意を得る' (to get consent).

その条件には同意しかねます。

I am unable to agree to those conditions.

Formal negative 'shikanemasu' (hard to do).

利用規約に同意してから登録してください。

Please register after agreeing to the terms of use.

Te-form + kara (after doing...).

彼は私の説明に同意したようだ。

It seems he agreed with my explanation.

Using 'yōda' (it seems).

会議で全員の同意を取り付けるのは難しい。

It is difficult to secure everyone's agreement in a meeting.

Phrase '同意を取り付ける' (to secure agreement).

手術には本人の同意が必要です。

The patient's own consent is required for the surgery.

Formal requirement context.

彼女が同意するまで待ちましょう。

Let's wait until she agrees.

Using 'made' (until).

プライバシーポリシーへの同意を求められた。

I was asked to agree to the privacy policy.

Passive form 'motomerareta' (was requested).

彼は渋々ながらも、その案に同意した。

He agreed to the plan, albeit reluctantly.

Using 'shibushibu nagara mo' (reluctantly).

株主の同意なしに決定はできない。

A decision cannot be made without the consent of the shareholders.

Noun '同意' + 'nashi ni' (without).

その理論に同意する専門家は少ない。

Few experts agree with that theory.

Relative clause modifying '専門家'.

事前に同意を得ておくことが重要だ。

It is important to obtain consent in advance.

Using 'te oku' (do in advance).

反対意見もあったが、最終的には同意に達した。

There were opposing opinions, but finally, an agreement was reached.

Phrase '同意に達する' (to reach agreement).

法的措置に同意する旨の書類を送った。

I sent a document stating my agreement to the legal action.

Using 'mune' (stating that...).

君が同意するしないに関わらず、計画は進む。

Regardless of whether you agree or not, the plan will proceed.

Structure 'suru shinai ni kakawarazu'.

インフォームド・コンセント、すなわち「説明と同意」が医療の基本だ。

Informed consent, namely 'explanation and agreement,' is the basis of medical care.

Academic definition context.

国民の広範な同意を得ることなく、法案を可決した。

The bill was passed without obtaining broad public consent.

Using 'kōhan na' (broad/wide-ranging).

黙示の同意があったとみなされる可能性がある。

There is a possibility that it will be considered that there was implied consent.

Legal term 'mokushi no dōi' (implied consent).

彼は自らの意志でその条件に同意したと主張している。

He claims that he agreed to those conditions of his own free will.

Using 'mizukara no ishi de' (of one's own will).

国際社会の同意を取り付けるための外交努力が続く。

Diplomatic efforts continue to secure the agreement of the international community.

High-level political context.

その解釈に同意するかどうかは、個々の判断に委ねられている。

Whether or not to agree with that interpretation is left to individual judgment.

Using 'yudanerarete iru' (is entrusted/left to).

契約内容を十分に理解した上で、同意ボタンを押した。

I pressed the agree button after fully understanding the contents of the contract.

Using 'ue de' (after/upon).

同意のプロセスにおいて、透明性が欠如していた。

In the process of agreement, transparency was lacking.

Abstract noun usage 'tōmeisei' (transparency).

当事者間の合意形成には、真摯な同意が不可欠である。

Sincere consent is essential for building consensus between the parties involved.

Using 'shinshi na' (sincere) and 'fukatsu' (indispensable).

憲法改正には国民投票による同意が必要とされる。

Consent via a national referendum is required for constitutional amendment.

Constitutional law context.

強要された同意は、法的に無効である。

Coerced consent is legally invalid.

Legal term 'kyōyō sareta' (coerced).

彼は沈黙をもって同意を示したと解釈された。

His silence was interpreted as showing consent.

Using 'motte' (by means of).

倫理委員会の同意を得るまで、研究は中断された。

The research was suspended until the consent of the ethics committee was obtained.

Institutional ethics context.

条約の批准には、議会の同意が前提条件となる。

The consent of the parliament is a prerequisite for the ratification of the treaty.

Diplomatic term 'hiju' (ratification).

自由かつ十分な情報に基づく同意（FPIC）の原則を遵守する。

Observe the principle of Free, Prior and Informed Consent (FPIC).

International human rights context.

同意の有無が、この裁判の最大の争点となっている。

Whether or not there was consent is the biggest point of contention in this trial.

Using 'umu' (existence or non-existence).

Common Collocations

全面的に同意する (completely agree)

意見に同意する (agree with an opinion)

提案に同意する (agree to a proposal)

条件に同意する (agree to conditions)

同意を得る (obtain consent)

同意を求める (seek agreement)

同意に達する (reach an agreement)

書面で同意する (agree in writing)

渋々同意する (reluctantly agree)

本人の同意 (the person's own consent)

Common Phrases

利用規約に同意する (Agree to terms of use)

同意しかねます (I am unable to agree)

ご同意いただけますか？ (Could I have your agreement?)

同意の上で (With consent/Upon agreement)

無断で (Without consent - antonymous phrase)

同意を取り付ける (To secure agreement)

説明と同意 (Informed consent)

同意を撤回する (To withdraw consent)

黙示の同意 (Implied consent)

同意が必要 (Consent is required)

Often Confused With

同意する vs 賛成 (Sansei)

Sansei is 'support/favor' (like a vote); Dōi is 'concurrence/consent' (like a contract).

同意する vs 了解 (Ryōkai)

Ryōkai is 'understood/roger'; Dōi is 'I agree with the logic/terms'.

同意する vs 納得 (Nattoku)

Nattoku is internal satisfaction/understanding; Dōi is external formal agreement.

Idioms & Expressions

Easily Confused

同意する vs

Sentence Patterns

How to Use It

nuance

'Dōi' is more about the 'mind' (intellect), while 'sansei' is more about the 'voice' (support).

formality

Always use 'dōi' in writing for terms and conditions; 'sansei' is too weak.

particle choice

While 'ni' is standard, 'to' is used when two parties agree with each other (A to B ga dōi suru).

Common Mistakes

Using '同意する' with friends for lunch.
Using the particle 'と' for an opinion.
Confusing '同意' with '了解'.
Saying '同意をあげる' (give agreement).
Thinking '同意' means 'I like it'.

Tips

Particle Ni

Always remember that the target of your agreement is marked with 'に'. This is different from English 'with'.

Consensus First

In Japan, formal '同意' often comes after informal 'Nemawashi'. Don't expect a 'yes' without prior discussion.

Polite Disagreement

Use '同意しかねます' instead of '同意しません' to sound more professional and less confrontational.

Checkbox Vocabulary

Look for '同意' on Japanese websites. It's the key to moving past registration screens.

Hospital Consent

If you are in a Japanese hospital, '同意書' is the form you sign to give permission for procedures.

Noun Phrases

Use '同意の上で' (upon agreement) to show that an action was taken with everyone's permission.

Adverb Boost

Add '全面的に' (completely) before '同意する' to show strong support in a formal debate.

Honorifics

If someone asks for 'ご同意' (go-dōi), they are showing high respect for your opinion.

Synonym Check

Remember: 'Sansei' is for voting/favor, 'Dōi' is for formal/legal consent.

Kanji Meaning

The kanji '同' (same) + '意' (mind) is a perfect literal translation of 'concurrence'.

Memorize It

Word Origin

A Sino-Japanese (Kango) word that has been used for centuries to describe intellectual or formal alignment.

Cultural Context

Japanese law places high importance on 'hanko' (seals) as a physical manifestation of '同意'.

The term 'Setsumei to Dōi' (Explanation and Consent) is the standard translation for 'Informed Consent' in Japanese hospitals.

In meetings, nodding (aizuchi) does not always mean 'I agree' (同意); it often just means 'I am listening.' Wait for the explicit word '同意' for legal certainty.

Practice in Real Life

Real-World Contexts

Conversation Starters

"私の提案に同意していただけますか？ (Could you agree to my proposal?)"

"この意見に同意する人は手を挙げてください。 (Those who agree with this opinion, please raise your hand.)"

"なぜその案に同意できないのですか？ (Why can't you agree to that plan?)"

"どちらの説に同意しますか？ (Which theory do you agree with?)"

"同意する前に、もう一度説明してください。 (Please explain one more time before I agree.)"

Journal Prompts

最近、何かに「同意」しましたか？ (Did you 'agree' to something recently?)

あなたが絶対に同意できないことは何ですか？ (What is something you can absolutely not agree with?)

日本の「同意」の文化についてどう思いますか？ (What do you think about the culture of 'agreement' in Japan?)

インターネットの利用規約を読んでから同意しますか？ (Do you read the terms of service before agreeing?)

仕事で同意を得るために、どのような工夫をしていますか？ (What steps do you take to get agreement at work?)

Frequently Asked Questions

10 questions

It is generally too formal for casual friends. If you use it, it might sound like you are joking or being intentionally stiff. For friends, '賛成' (sansei) or 'いいよ' (ii yo) is much better. Use '同意する' when discussing serious topics like contracts, medical decisions, or formal debates.

'同意' (dōi) is often one-sided consent or alignment (I agree to your terms). '合意' (gōi) usually implies a mutual agreement reached after negotiation (We both agreed on a settlement). You see '合意' more in legal settlements and '同意' in permission forms.

In Japan, '利用規約に同意する' (Agree to Terms of Use) is the legal standard. It signifies that the user has formally accepted the rules of the service. It is the equivalent of the English 'I agree' button.

Not usually. For feelings, '同感' (dōkan - same feeling) or '共感' (kyōkan - empathy) is used. '同意' is more about opinions, plans, and legal consent. You wouldn't say 'I agree with your sadness' using '同意'.

The most common particle is 'に' (ni). For example, '彼の意見に同意する' (I agree to his opinion). If you are agreeing 'with' a person as a partner in a mutual agreement, you might use 'と' (to), but 'に' is the standard for the object of agreement.

In a professional setting, you can say 'その点については同意しかねます' (I am unable to agree on that point). 'しかねる' is a formal way to say 'cannot' that sounds more polite than 'できません'.

It is both. '同意' (dōi) is the noun meaning 'agreement' or 'consent.' By adding 'する' (suru), it becomes the verb 'to agree.' You can use the noun in phrases like '同意を得る' (to get consent).

It is often called 'インフォームド・コンセント' (infōmudo konsento) or the more traditional Japanese translation '説明と同意' (setsumei to dōi), which literally means 'explanation and agreement.'

Yes, in a linguistic context, '同意語' (dōigo) means 'synonym' (words with the same meaning). This comes from the same kanji '同' (same) and '意' (meaning).

Yes, '婚姻の同意' (consent to marriage) is a legal term. In traditional contexts, getting the '同意' of the parents was a crucial step in the marriage process.