Grammar Rule in 30 Seconds
Use 'must' for personal feelings of necessity and 'have to' for outside rules or facts.
- Use 'must' for internal obligation: 'I must study harder.'
- Use 'have to' for external rules: 'I have to wear a uniform at work.'
- Negative 'mustn't' means 'don't do it', but 'don't have to' means 'it's optional'.
Overview
mustとhave toの使い分けは非常に重要です。どちらも日本語では「～しなければならない」と訳されますが、その本質的なニュアンスは大きく異なります。日本語では「しなければならない」という一つの表現で、義務、強制、あるいは自分の意志による決意までカバーしますが、英語では「義務の発生源」によってこれらを明確に使い分けます。mustとhave toの最大の違いは、義務の「発生源」です。mustは「話し手の主観的な強い思い」や「内面的な義務」を表します。一方、have toは「外部からのルール」「客観的な状況」「避けられない事情」など、話し手の意志とは関係のない外部要因による義務を表します。mustは、日本語の「～するべきだ」「～しなくてはならない（自分の意志）」に近く、have toは「～する決まりだ」「～せざるを得ない（状況的に）」に近いと言えます。You have to wear a mask となります。もし店員さんが You must wear a mask と言うと、店員さん個人の強い命令や、非常に高圧的な響きを与えてしまう可能性があります。このように、英語では「誰のルールなのか」を意識することで、相手に与える印象をコントロールできます。mustは助動詞なので主語によって形が変わりませんが、have toは一般動詞の性質を持つため、主語が三人称単数の場合は has to に変化します。I must study harder.(自分の意志で「もっと勉強しなきゃ」)I have to study for the exam.(試験があるという外部要因で「勉強しなきゃ」)You must not smoke here.(「ここでタバコを吸ってはいけない」という強い禁止)You don't have to pay.(「支払う必要はない＝払わなくてもいい」という選択肢の提示)
mustは「強い確信」や「個人的な重要性」を伝える際に使われます。例えば、友人に「この映画は絶対に見るべき！」と勧めたいときは You must see this movie! と言います。これは「絶対に見る価値がある」というあなたの個人的な強い推奨です。また、論理的な推論（～に違いない）としても使われます。例えば、雨が降った後の濡れた道路を見て It must have rained.（雨が降ったに違いない）と言うのは、状況証拠から確信している状態です。have toは「仕事の締め切り」「法的なルール」「電車の時刻」など、個人の気持ちでは変えられない事実に使います。就活の面接で「なぜこの会社を選んだのですか？」と聞かれた際、I have to work here と言ってしまうと「（生活のために）ここで働かざるを得ない」という消極的な意味になりかねません。状況に応じて、自分の主体的な意志を示すか、客観的な義務を示すかを使い分けることが、プロフェッショナルな英語表現の鍵となります。- 1
mustn'tとdon't have toの混同
mustn't を使ってしまいます。mustn't は「～してはいけない（禁止）」です。例えば、「明日、学校に行かなくていいよ」と言いたいとき、You mustn't go to school と言うと「学校に行ってはいけない（禁止）」という恐ろしい意味になります。正しくは You don't have to go to school です。これは日本語の「～しなくていい」という表現が「禁止」と「不要」の両方を曖昧に含んでしまうため、L1干渉が起きやすい箇所です。- 1過去形での
mustの誤用
must には過去形がありません。昨日の義務を話すとき、I must go to the doctor yesterday と言うのは間違いです。過去の義務はすべて had to を使います。日本語では過去の義務も現在と同じ「しなければならない」を使うため、英語でも must をそのまま使ってしまうミスが多発します。- 1疑問文での
mustの使用
Must you go? と聞くと、非常に威圧的で「どうしても行かなきゃいけないのか？」という強い口調になります。日常会話で「～しなければなりませんか？」と尋ねる場合は、常に Do you have to...? を使うのが自然です。これも日本語の「～しなければなりませんか？」という丁寧な表現をそのまま直訳しようとする思考が原因です。should は must よりもずっと弱く、相手に選択の余地を残したアドバイスです。need to は have to に近いですが、義務というよりは「必要不可欠なこと」に焦点が当たります。まずは must と have to の違いを軸に、これらを使い分ける練習をしてみましょう。have to を使って「～に違いない」と言えますか？must を使います。have to はあくまで義務や必要性を表す言葉です。must はカジュアルな会話で使ってもいいですか？have to や need to を使う方が自然で、こなれた印象を与えます。has to と have to の使い分けが難しいです。he, she, it (三人称単数) のときだけ has to になります。それ以外はすべて have to です。これは一般動詞の have の活用と同じルールなので、難しく考えすぎないでください！2. Contractions and Negatives
| Full Form | Contraction | Meaning |
|---|---|---|
|
must not
|
mustn't
|
Forbidden / Prohibition
|
|
do not have to
|
don't have to
|
Optional / Not necessary
|
|
does not have to
|
doesn't have to
|
Optional (3rd person)
|
|
did not have to
|
didn't have to
|
Not necessary in the past
|
Conjugating Must and Have to
| Subject | Must (Modal) | Have to (Semi-Modal) | Past (Obligation) |
|---|---|---|---|
|
I
|
must
|
have to
|
had to
|
|
You
|
must
|
have to
|
had to
|
|
He/She/It
|
must
|
has to
|
had to
|
|
We
|
must
|
have to
|
had to
|
|
They
|
must
|
have to
|
had to
|
Meanings
Both 'must' and 'have to' express that something is necessary or required, but they differ in the source of the obligation.
Strong Personal Obligation
When the speaker decides something is necessary for themselves.
“I must get more sleep.”
“We must try that new restaurant.”
External Obligation
When a rule, law, or another person makes something necessary.
“Students have to arrive on time.”
“Do you have to work on Saturdays?”
Prohibition
Using 'must not' or 'mustn't' to say something is forbidden.
“You mustn't smoke here.”
“Visitors must not touch the paintings.”
Lack of Necessity
Using 'don't have to' to say something is optional.
“You don't have to come if you're tired.”
“We don't have to wear a tie at the office.”
Reference Table
| 視点 | Must | Have to | Example |
|---|---|---|---|
|
義務の源
|
Internal (speaker's feeling)
|
External (rules, laws, circumstances)
|
I `must` finish this. vs. I `have to` finish this.
|
|
否定形
|
`Mustn't` (Prohibition)
|
`Don't/Doesn't have to` (Lack of necessity)
|
You `mustn't` park here. vs. You `don't have to` park here.
|
|
過去形
|
(なし) → `had to` で代用
|
`Had to`
|
I `had to` leave early.
|
|
質問
|
Rare/Formal (`Must I...?`)
|
Common (`Do/Does...have to...?`)
|
`Do you have to` work today?
|
|
フォーマルさ
|
More formal, emphatic
|
More common, flexible
|
You `must` see this! vs. You `have to` see this!
|
|
基本的な構造
|
Subject + `must` + base verb
|
Subject + `have/has to` + base verb
|
She `must go`. vs. She `has to go`.
|
フォーマル度スペクトル
It is imperative that we depart immediately. (Leaving a location)
We must go now. (Leaving a location)
We have to go. (Leaving a location)
We gotta bounce. (Leaving a location)
Must と Have to: 核となる違い
Must
- 内面的な感情 話し手の信念
- 強いアドバイス 個人的な推奨
- 禁止 Mustn't (禁止されている)
Have To
- 外部のルール 法律、上司、状況
- 一般的な使い方 日常的な義務
- 不必要性 Don't have to (任意)
Must と Have To の比較
Must または Have To の選択フローチャート
その義務は外部のルール、法律、または状況によるものですか？
それは、あなたからの強い個人的な感情、信念、または推奨ですか？
何かが禁止されている、または禁じられていることについてですか？
何かが不必要である、または任意であることについてですか？
必要性のスペクトラム
強い個人的な動機
- • I must apologize.
- • You must try this!
外部からの要求
- • I have to be at work by 9.
- • Students have to wear uniforms.
禁止
- • You mustn't touch.
- • We mustn't be late.
任意
- • You don't have to pay.
- • I don't have to rush.
レベル別の例文
I must go now.
I need to leave.
I have to work today.
It is my job to work.
You must eat your vegetables.
It is important to eat them.
Do I have to go to bed?
Is it required?
You mustn't park here.
Parking is forbidden.
She doesn't have to cook tonight.
Cooking is optional.
We had to wait for an hour.
It was necessary in the past.
He has to wear a suit at work.
It is a company rule.
You must see the new exhibition; it's amazing!
I strongly recommend it.
I've got to go, my bus is leaving.
Informal necessity.
You don't have to bring anything to the party.
No obligation.
Must we finish this today?
Formal inquiry about necessity.
The light is on, so they must be home.
I am almost certain they are home.
You must not disclose this information.
Confidentiality is required.
I didn't have to take the exam because of my grades.
Exemption from obligation.
Does he have to be so loud?
Annoyance at a behavior.
All applicants must submit their portfolios by Friday.
Official requirement.
One must consider the environmental impact.
General formal necessity.
You needn't have worried about the meeting.
It wasn't necessary, but you did it anyway.
It must be noted that the results are preliminary.
Formal emphasis.
If you must know, I'm leaving because I'm bored.
Sarcastic/reluctant necessity.
The law stipulates that citizens must be informed.
Legal mandate.
He has to have been lying all along.
Strong deduction about the past.
Must you always be so contrarian?
Rhetorical question expressing frustration.
間違えやすい
Learners use 'must' when they only want to give a suggestion.
This is the most common error. Learners think 'mustn't' means 'not necessary'.
Learners don't know when to use the informal 'gotta'.
よくある間違い
I must to go.
I must go.
He musts study.
He must study.
I don't must go.
I mustn't go / I don't have to go.
Must you to stay?
Must you stay?
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.
Do you must go?
Do you have to go? / Must you go?
She has must to go.
She has to go.
I must have gone yesterday.
I had to go yesterday.
You don't have to smoke here.
You mustn't smoke here.
I must to finish my work.
I must finish my work.
Must I to bring a gift?
Do I have to bring a gift?
It's a must-to-have item.
It's a must-have item.
You mustn't have worried.
You needn't have worried.
I had must to do it.
I had to do it.
Must you to be so loud?
Must you be so loud?
文型パターン
I have to ___ because ___.
You mustn't ___ or you will ___.
In my country, you don't have to ___ but you should ___.
If we want to succeed, we must ___.
Real World Usage
You have to take off your shoes.
You must finish the whole course of antibiotics.
I've gotta go, see ya!
Will I have to travel for this role?
You MUST try this new filter!
Drivers must stop for pedestrians.
You have to enter a valid address.
Internal vs. External
Must は自分の気持ちや信念からくる義務を表すことが多いです。一方、have to は外部のルール、人、状況による義務を示します。例えば、心の中で「もっと勉強しなきゃ」と思うときは、「I must study harder.」Mind the Negatives!
Past Tense Savior
must は使えません！had to があなたのベストフレンドです。例えば、「昨日遅くまで仕事をしなくてはいけなかった」は、「I had to work late yesterday.」 と言います。I musted work late.は間違いです。
Sounding Natural
have to を使うのが一般的です。Must は少しフォーマルすぎたり、ドラマチックに聞こえることがあります。友達と話すとき、「明日早いから早く寝なきゃ」は「I have to go to bed early.」 が自然です。Questions with 'Have to'
do または does を have to と一緒に使います。「身分証明書を持ってこないといけませんか？」と聞くときは、「Do I have to bring my ID?」 と言います。Must I bring my ID?はフォーマルすぎて、あまり使いません。
Smart Tips
Use 'must' to sound more enthusiastic and warm.
Always switch to 'had to'. Don't even think about 'must'.
Think 'mustn't'. It helps you associate the word with a hard stop.
Use 'have to' for rules and 'gotta' for urgent needs.
発音
Mustn't Silent T
The first 't' in 'mustn't' is almost always silent in natural speech.
Have to -> Haftu
In fast speech, 'have to' sounds like 'haftu' and 'has to' sounds like 'hastu'.
Gotta
In very informal American English, 'have got to' is reduced to 'gotta'.
Emphasis on Must
You MUST see it!
Conveys strong personal recommendation or urgency.
Rising intonation on questions
Do I have to? ↗
Conveys reluctance or checking a rule.
暗記しよう
記憶術
Must is for ME (internal), Have to is for the HOUSE (external rules).
視覚的連想
Imagine a heart for 'must' (internal feeling) and a heavy law book for 'have to' (external rules).
Rhyme
Must is what I feel inside, Have to is what the rules decide.
Story
A chef says 'I must cook' because he loves it. But at the restaurant, he 'has to cook' because customers are waiting. If he 'mustn't cook', the kitchen is closed. If he 'doesn't have to cook', he can take a break.
Word Web
チャレンジ
Write down 3 things you MUST do for your health and 3 things you HAVE TO do for your job/school today.
文化メモ
British speakers use 'must' more frequently for personal obligations than American speakers.
Americans almost exclusively use 'have to' or 'have got to' in speech. 'Must' can sound overly formal or dramatic.
In legal documents, 'shall' was traditionally used, but 'must' is now preferred to clearly indicate a requirement.
'Must' comes from the Old English 'moste', which was actually the past tense of 'motan' (to be allowed/able).
会話のきっかけ
What are three things you have to do every morning?
Do you have to wear a uniform for your job or school?
If you could change one law, what would people not have to do anymore?
What are some things visitors mustn't do when they visit your country?
日記のテーマ
よくある間違い
Test Yourself
You ___ forget to lock the door when you leave.
Choose the correct sentence:
Find and fix the mistake:
I musted go to the dentist yesterday.
Must には義務を表す過去形がありません。過去の必要性には had to を使います。Translate into English: 'No tienes que venir si no quieres.'
Answer starts with: ["Y...
Score: /4
練習問題
8 exercisesAll passengers ___ show their passports at the gate.
She ___ (work) late every Tuesday.
Find and fix the mistake:
You mustn't pay for the water; it is free.
I must finish my homework today.
A: Is the museum free? B: No, you ___ buy a ticket.
Select the correct sentence.
1. I must go. 2. I don't have to go. 3. I mustn't go.
You've been working for 12 hours! You ___ be exhausted.
Score: /8
Practice Bank
12 exercisesHe ___ wear a uniform at his new job.
I really ___ finish this book, it's so good!
My boss said we ___ be late for the meeting tomorrow.
She doesn't must go to the office today.
Do you must work on weekends?
Choose the correct sentence:
Choose the correct sentence:
Translate into English: 'Ella tuvo que esperar mucho tiempo.'
Translate into English: 'No debes tocar la obra de arte.'
Arrange these words into a sentence:
Arrange these words into a sentence:
Match the phrase with its meaning.
Score: /12
よくある質問 (8)
No, not for obligation. You must use `had to`. If you use `must have`, it means you are making a guess about the past (e.g., 'He must have been tired').
Actually, `must` can sound more bossy or aggressive in some contexts. `Have to` is usually more neutral because it sounds like the rule comes from somewhere else, not just you.
It is an informal version of `have to`. 'I've got to go' is very common in spoken English and is often shortened to 'I gotta'.
It exists, but it is very rare in speech. Americans usually say 'You can't' or 'You're not allowed to' instead of `mustn't`.
`Must` is a modal verb, and modal verbs never change their form. `Have to` is a semi-modal and follows regular verb rules for the third person.
Yes, but it sounds very formal. 'Must you leave so soon?' is correct but sounds like a movie from the 1940s. 'Do you have to leave?' is more modern.
They are very similar. `Need to` focuses more on the necessity for the person, while `have to` focuses more on the requirement or rule.
They are often used for the same thing (prohibition). `Mustn't` is more formal and authoritative; `can't` is more common in daily speech.
Scaffolded Practice
1
2
3
4
Mastery Progress
Needs Practice
Improving
Strong
Mastered
In Other Languages
tener que / deber
Spanish uses the same verb for 'must' and 'should' (deber), just in different moods.
devoir / il faut
French doesn't distinguish between internal and external obligation with different verbs.
müssen / nicht dürfen
The negative 'muss nicht' is a 'false friend' for English 'mustn't'.
~nakereba naranai
Japanese uses conditional grammar rather than modal verbs.
yajib an (يجب أن)
Arabic uses an impersonal verb rather than a modal that conjugates with the subject.
děi (得) / bìxū (必须)
Chinese verbs do not conjugate, so the complexity of 'has to' vs 'have to' is absent.
Learning Path
Prerequisites
Continue With
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