Grammar Rule in 30 Seconds
Modal verbs like 'can', 'should', and 'must' change the mood of a sentence without ever changing their own form.
- Never add -s, -ed, or -ing to a modal verb. Example: 'He can swim' NOT 'He cans swim'.
- Always follow a modal with the base verb (no 'to'). Example: 'I must go' NOT 'I must to go'.
- To make a question, just swap the subject and the modal. Example: 'Should I stay?'
Overview
he walks なのか they walk なのかを覚えたり、 did や does を気にしたりしなければなりません。助動詞（Modal verbs）はあなたの新しい親友です。これらは怠け者の夢です。誰に対しても形を変えません。 -s も -ed もなく、ドラマもありません。ただそこにいて、文章に「スパイス」を加えるだけです。動詞の絵文字（emoji）のようなものだと考えてください。I go は事実です。 I can go はニュアンスです。助動詞は can, should, must のような特別な動詞です。能力、許可、必要性などを表現します。英語における究極のショートカットです。動詞がビデオゲームだとしたら、助動詞はチートコードです。余計な活用なしに、スピーチをより繊細にします。気づかないうちに一日に100回は使っているでしょう。Zoom会議での Can you hear me?は典型的な助動詞の瞬間です。
Should I post this selfie?もそうです。現代生活のいたるところにあります。Netflixの字幕からWhatsAppのグループチャットまで、助動詞が重労働をこなします。丁寧、自信、あるいは慎重な印象を与えるのに役立ちます。これらをマスターすることは、流暢さの新しいレベルをアンロックするようなものです。リストを暗記することよりも、気分（mood）を感じることの方が重要です。
How This Grammar Works
I can と言うことはできません。まあ、「誰が助けられる？」と聞かれたら言えますが、通常はパートナーが必要です。そのパートナーは主要な動作動詞です。これらは主要な動詞の意味を修正して、異なる「気分」を示します。Instagramの写真のフィルターのようなものだと考えてください。写真（主要な動詞）はそのままです。フィルター（助動詞）が人々の見え方を変えます。 I eat はただの事実の表明です。 I might eat は、まだUber Eatsのメニューをチェックしているように聞こえます。I must eat は、長時間のジムセッションの後に餓死しそうに聞こえます。これらの動詞は独自のルールに従います。 play や eat のような「普通の」動詞のようには振る舞いません。これらは反逆者です。主要な動詞の前に to という言葉を使いません。 I can to swim とは決して言いません。これは聞き手にとって大きな警告信号です。また、 -ing 形もありません。辞書で musting や shoulding を見ることは決してありません。固定されていて安定しています。いつも同じバーガーを注文する友人のようなものです。
Formation Pattern
I, you, we、あるいは my cat でも構いません。
can, could, may, might, shall, should, will, would, must から選びます。
-s も -ed も -ing も付けません。
She can speak English.
she であっても can に -s が付かないことに注目してください。
speak の前に to がないことに注目してください。
not を追加するだけです。
I cannot (can't) go.
You should not (shouldn't) do that.
Can you help me?
do, does, did を決して使わないでください。
Do you can help?
Can you help?
do ファミリーの助けを必要としません。文法界の独立したインフルエンサーです。
When To Use It
- 一般的な能力には
canを使います。I
cancode. - 許可には
canやmayを使います。CanI use your charger? - 強い義務には
mustを使います。You
mustwear a seatbelt. - アドバイスを与えるには
shouldを使います。You
shouldwatch that new Netflix show. - 可能性には
mightやcouldを使います。It
mightrain later. - 現代のテキストメッセージでは、
canが何にでもよく使われます。しかし、mayやcouldを使うと、ずっとプロフェッショナルに聞こえます。教授にメールを送る場面を想像してください。Can I ask a question?もいいですが、Could I ask a question?の方が、ちゃんと予習をしたように聞こえます。オンラインゲームをしているなら、チームにアドバイスするためにWe
と言うかもしれません。旅行のVlogを撮っているなら、興奮を示すためにshouldrush B!You
と言うかもしれません。各助動詞には、RPGのキャラクターのように特定の役割があります。mustvisit this café!
Common Mistakes
- 助動詞の後に
toを入れる。これが一番の間違いです。I must to studyとは決して言わないでください。 - 三人称に
-sを付ける。He cansは間違いです。常にHe canです。 - 疑問文で
doを使う。Does he can?と言うと、人々は首をかしげるでしょう。 - 2つの助動詞を一緒に使おうとする。
I will can goは大きな間違いです。代わりにI will be able to goを使ってください。 mustが非常に強いことを忘れる。アドバイスに使うと威圧的に聞こえることがあります。- 丁寧な依頼で
canとcouldを混同する。見知らぬ人にはcouldの方が常に安全です。 - 「～する必要はない」と言いたい時に
mustn'tを使う。mustn'tは「禁止」を意味します。
Contrast With Similar Patterns
MustvsHave to:Mustは個人的な感情、Have toは外部のルールであることが多いです。ShouldvsOught to: 意味は同じです。Shouldの方がずっと一般的です。CanvsBe able to:Canは現在、Be able toは未来や過去に使います。MayvsMight:Mayの方がMightよりも少し可能性が高いです。
Quick FAQ
助動詞は常に同じですか？ A: はい、形は決して変わりません。
過去のことに can を使えますか？ A: いいえ、過去の能力には could を使います。
should は must より強いですか？ A: いいえ、 must は命令で、 should は単なる提案です。
ought to には to が必要ですか？ A: はい、 ought to だけが to を使います。
どうやって全部覚えますか？ A: 気分転換（mood-shifters）だと考えてください。文章のエネルギーを変えるものです。
Modal Verb Conjugation (No Change!)
| Subject | Modal | Base Verb | Notes |
|---|---|---|---|
|
I
|
can / should / must
|
work
|
No 'to'
|
|
You
|
can / should / must
|
work
|
Standard form
|
|
He / She / It
|
can / should / must
|
work
|
NO '-s' added
|
|
We
|
can / should / must
|
work
|
Standard form
|
|
They
|
can / should / must
|
work
|
Standard form
|
Common Contractions
| Full Form | Contraction | Usage |
|---|---|---|
|
cannot
|
can't
|
Very common in speech
|
|
should not
|
shouldn't
|
Common for advice
|
|
must not
|
mustn't
|
Used for prohibitions
|
|
shall not
|
shan't
|
Very rare/British
|
Meanings
Modal verbs are a special category of auxiliary verbs used to express ability, possibility, permission, advice, or necessity. They function as 'mood setters' for the main verb that follows them.
Ability & Permission (Can)
Used to express physical or mental ability, or to ask for/give permission in informal settings.
“I can run a marathon in under four hours.”
“Can I borrow your pen for a second?”
Advice & Recommendation (Should)
Used to give suggestions, advice, or to talk about what is ideal or expected.
“You should try the pasta; it's delicious here.”
“We should leave now if we want to catch the train.”
Strong Obligation (Must)
Used to express a firm necessity, a rule, or a personal strong feeling that something is required.
“All passengers must wear a seatbelt.”
“I must remember to call my mother tonight.”
Logical Deduction (Must)
Used when you are almost 100% sure that something is true based on evidence.
“You've been working all day; you must be exhausted.”
“The lights are off; they must be asleep.”
Reference Table
| 助動詞 | 意味 | 例文（肯定） | 例文（否定） |
|---|---|---|---|
|
can
|
能力・許可・可能性
|
I `can` speak French.
|
I `can't` speak French.
|
|
should
|
助言・勧め・予測
|
You `should` rest.
|
You `shouldn't` worry.
|
|
must
|
強い義務・必要性・推量
|
We `must` leave now.
|
You `must not` smoke here.
|
|
could
|
過去の能力・丁寧な依頼
|
I `could` sing well then.
|
`Could` you open the door?
|
|
have to
|
外的義務・必要性
|
I `have to` work late.
|
I `don't have to` go.
|
|
mustn't
|
禁止
|
You `mustn't` touch that.
|
|
フォーマル度スペクトル
May I please have some water? (Restaurant/Home)
Can I have some water, please? (Restaurant/Home)
Can I get a water? (Restaurant/Home)
Gimme a water, yeah? (Restaurant/Home)
助動詞：Can, Should, Must
Can
- 能力 I can sing.
- 許可 You can leave.
- 可能性 It can be cold.
Should
- 助言 You should rest.
- 勧め We should go.
- 予測 They should arrive.
Must
- 義務 You must study.
- 必要性 We must hurry.
- 推量 He must be tired.
must not と don't have to の比較
正しい助動詞の選び方 (Can, Should, Must)
能力や許可についてですか？
アドバイスやおすすめですか？
強い義務や論理的な確信ですか？
助動詞のパワーアップ！
能力を表現する
- • I can run fast.
- • She can solve this problem.
アドバイスをする
- • You should eat healthy.
- • He should apologize.
義務を述べる
- • We must follow rules.
- • Students must study.
丁寧な依頼
- • Can you help me?
- • Could you close the door?
レベル別の例文
I can play the guitar.
Can you help me?
She can speak English.
We can't come to the party.
You should eat more vegetables.
I must finish my homework.
Should we go to the park?
You mustn't touch that; it's hot!
You should try to arrive ten minutes early.
He must be the new manager; he's wearing a suit.
You don't have to pay now; you can pay later.
Can I suggest a different approach?
The government should have acted much sooner.
There must have been a misunderstanding at the front desk.
You can't have seen Mark; he's in Australia!
Must you make so much noise while I'm working?
One should always consider the ethical implications of such research.
The results can be interpreted in several different ways.
It must be noted that this study has several limitations.
Should you require further assistance, do not hesitate to contact us.
The sheer audacity of the claim must surely give us pause.
One cannot but admire her dedication to the cause.
The policy should, in theory, alleviate the housing crisis.
That he should have failed after such effort is a tragedy.
間違えやすい
Learners often think 'mustn't' is the negative of 'must' in terms of necessity, but it actually changes the meaning to prohibition.
Both are used for permission, but 'can' is informal and 'may' is formal. Teachers often correct 'Can I...?' with 'I don't know, can you?' to emphasize ability.
They mean the same thing, but 'ought to' is much less common and requires 'to'.
よくある間違い
I can to swim.
I can swim.
He cans speak English.
He can speak English.
I no can go.
I can't go.
Do you can help?
Can you help?
You should to rest.
You should rest.
Must you to go?
Must you go?
She shoulds study.
She should study.
You mustn't pay; it's free.
You don't have to pay; it's free.
I musted work yesterday.
I had to work yesterday.
It should be rain tomorrow.
It should rain tomorrow.
I can't help but to wonder.
I can't help but wonder.
He must has forgotten.
He must have forgotten.
Should I'll see him, I'll tell him.
Should I see him, I'll tell him.
文型パターン
I can ___ but I can't ___.
If you are tired, you should ___.
To be a good student, you must ___.
He isn't answering his phone; he must be ___.
You shouldn't have ___ without ___.
Real World Usage
I can work under pressure and I should be able to start next month.
Can u come tonight? You should definitely be there!
Passengers must present their boarding pass.
You should take this medicine twice a day.
You MUST try this new brunch spot! 😍
Can I get the burger without onions?
We should schedule a meeting to discuss the budget.
The battery must be charged for 4 hours before use.
助動詞＋動詞の原形＝魔法！
mustn't と don't have to の違い
don't have to means 'it's not necessary' (you can choose not to)."丁寧な「Could」を使ってみよう
イギリス英語とアメリカ英語の「must」
In British English, must for obligation is often a bit more common than have to in certain contexts.助動詞は変化しない！
Smart Tips
Stop! Imagine the modal verb is a 'to-eater'. It eats the 'to' before it can reach the main verb.
Use 'might want to' or 'should perhaps' instead of just 'should' to sound more professional.
It's probably a logical guess, not an obligation. 'It must be raining' doesn't mean the rain is following a rule!
Avoid contractions like 'can't' or 'shouldn't'. Use the full forms 'cannot' and 'should not'.
発音
The Silent 'L'
In 'should', the 'l' is completely silent. It sounds like 'shood'.
Can vs. Can't
In American English, 'can' is often reduced to 'kun', while 'can't' has a clear 'a' sound and a sharp 't' (or a glottal stop).
Mustn't
The first 't' in 'mustn't' is silent. It sounds like 'muss-unt'.
Polite Requests
Can you ↗ help me?
Rising intonation at the end makes the request sound more polite.
Strong Advice
You ↘ SHOULD go.
Stress on the modal verb 'should' emphasizes the importance of the advice.
暗記しよう
記憶術
Modals are 'Bare' and 'Fixed': they use the Bare infinitive and their form is Fixed (no -s).
視覚的連想
Imagine a 'Modal Shield' around the main verb. The shield blocks the 'to' and the 's' from ever touching the verb.
Rhyme
With can, should, and must, the 'to' you must bust; no 's' at the end, they're a learner's best friend.
Story
A king (Must) gives orders, a wise advisor (Should) gives suggestions, and a skilled knight (Can) shows what he is able to do. They all live in a castle where the word 'to' is forbidden.
Word Web
チャレンジ
Write three sentences about your job or hobby: one thing you CAN do, one thing you SHOULD do better, and one thing you MUST do every day.
文化メモ
British speakers often use 'should' in places where Americans might use 'would' or 'must', especially to sound more polite or indirect.
Americans rarely use 'mustn't' in casual speech, preferring 'can't' or 'not allowed to'. 'Mustn't' can sound very formal or old-fashioned.
Using 'should' instead of 'must' in a business meeting is a common 'hedging' strategy to avoid sounding too bossy to colleagues.
Modal verbs come from Old English 'pre-present' verbs, which were originally past tense forms that took on a present tense meaning.
会話のきっかけ
What is one skill you can do really well?
What should a tourist see in your hometown?
What are some things you must do before you turn 30?
If you could change one law in your country, what should it be?
What must have happened for the world to end up in its current state?
日記のテーマ
よくある間違い
Test Yourself
She ___ speak three languages fluently.
Find and fix the mistake:
You shouldn't to worry about the exam.
Choose the correct sentence:
Translate into English: 'No debes usar tu teléfono durante el examen.'
Answer starts with: ["Y...
Score: /4
練習問題
8 exercisesYou ___ see the new Batman movie; it's great!
She can ___ (speak) three languages fluently.
Find and fix the mistake:
He musts finish his report by tomorrow.
I should call him. -> ___
1. Can, 2. Should, 3. Must
A: Is that the doorbell? B: Yes, it ___ be the pizza delivery guy.
Select the right one:
The past tense of 'must' is 'musted'.
Score: /8
Practice Bank
11 exercisesThey ___ finish their project by Friday.
He can to swim very well.
Choose the correct sentence:
Translate into English: 'Debes estudiar para el examen.'
Arrange these words into a sentence:
Match the modals with their primary function:
We ___ respect other cultures when traveling.
She must to be tired after that long flight.
Choose the correct sentence:
Translate into English: '¿Puedes ayudarme con mi tarea?'
Arrange these words into a sentence:
Score: /11
よくある質問 (8)
Yes, in informal situations. For example, `Can I use your phone?` is perfectly fine with friends. In formal settings, use `May I...` instead.
They mean the same thing. `Cannot` is the formal, written version, and `can't` is the everyday spoken version. Note that `cannot` is always one word.
Yes, much stronger. `Should` is a suggestion (you have a choice), while `must` is an obligation (you have no choice).
No. Modals cannot be used together. To express future ability, use `I will be able to`.
This is a common mistake made by native speakers because `should've` sounds like `should of`. It is always grammatically incorrect to write `should of`.
Use it for things that are strictly forbidden, like `You mustn't park here`. It is more common in British English than American English.
Yes! You can say `He should be here soon` if you expect him to arrive based on a schedule.
No. There is no such word as `canning`, `shoulding`, or `musting` in English grammar.
Scaffolded Practice
1
2
3
4
Mastery Progress
Needs Practice
Improving
Strong
Mastered
In Other Languages
poder / deber / tener que
Spanish modals are followed by an infinitive (with no 'to' equivalent), but they conjugate fully unlike English modals.
pouvoir / devoir
French uses the conditional tense of 'devoir' (devrais) to express 'should', whereas English has a dedicated word.
können / sollen / müssen
German modals still conjugate for person (ich kann, du kannst), while English ones are fixed.
~reru / ~nakereba naranai
Obligation in Japanese is often expressed as a double negative ('if you don't do it, it won't become').
yumkin / yajib
The 'modal' part doesn't change, but the following verb must conjugate for the subject.
néng (能) / yīnggāi (应该) / bìxū (必须)
Chinese doesn't have the 'no-to' or 'no-s' problem because Chinese verbs never change form anyway.
Learning Path
Prerequisites
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