What are Modals of Obligation?

Modals of obligation are verbs that show how necessary or important an action is. They come before the main verb in a sentence.

These verbs help us express rules, duties, advice, and things that are not allowed or not necessary.

• Must そして have to show strong obligation or necessity.
• Mustn’t そして can’t show prohibition (something is not allowed).
• Don’t have to shows a lack of obligation (something is not necessary).
• Should そして ought to give advice or recommendations.

When to Use Must そして Have To?

Must そして have to both express obligation, but there is a small difference in how we use them.

• 当社では must when the speaker feels the obligation strongly. It often comes from an internal feeling or a rule the speaker makes for themselves.
• 当社では have to when the obligation comes from outside, like a rule, law, or another person's authority.

In spoken English, have to is more common than must for general obligations.

Talking About Things That Are Not Allowed: Mustn't そして Can't

When something is forbidden or not permitted, we use mustn't または can't.

• Mustn't （または must not) is used for strong prohibitions, often from the speaker or a very important rule.
• Can't （または cannot) also shows prohibition, often referring to rules made by others or general impossibility.

When Something Is Not Necessary: Don't Have To

If there is no obligation to do something, or it is not necessary, we use don't have to.

• This means you can do it if you want, but it's not required.
• It is very different from mustn't, which means you are not allowed to do something.

Giving Advice and Recommendations: Should そして Ought To

To give advice or suggest something is a good idea, we use should または ought to.

• These modals show a weaker obligation than must または have to.
• Should is much more common than ought to, especially in everyday conversation.
• The negative forms are shouldn't そして ought not to.

Past and Future Obligations

The modal must does not have a past or future form. For past and future obligations, we always use have to.

• For the past, use had to.
• For the future, use will have to.

例

• I must study for my exam tonight. (Personal necessity)
• You have to wear a seatbelt in the car. (External rule/law)
• We mustn't make noise after 10 PM. (Strong prohibition)
• You can't park here; it's a no-parking zone. (External rule/prohibition)
• She doesn't have to work on Sundays. (Not necessary)
• You should visit the museum; it's fantastic. (Advice)
• They ought to apologize for their behavior. (Recommendation)
• Yesterday, I had to finish my report. (Past obligation)
• Tomorrow, we will have to wake up early. (Future obligation)

練習

スペイン語話者がよく犯す間違い

Spanish speakers often make mistakes with modals of obligation, especially confusing prohibition and lack of necessity.

• Mustn't 対. Don't have to: A common error is using don't have to when you mean mustn't. In Spanish, "no tener que" means "not have to" (no obligation), but sometimes learners use it incorrectly to mean "it's forbidden."
  Incorrect: You don't have to smoke here. (This means 'it's not necessary to smoke here', which is strange.)
  Correct: You mustn't smoke here. (This means 'it's forbidden to smoke here.')
• Must 対. Have to for external rules: 一方で must can sometimes be used for external rules, have to is generally more natural for obligations imposed by others or by law.
  Less natural: You must buy a ticket before boarding.
  More natural: You have to buy a ticket before boarding.
• 使用方法 must in the past/future: Remember that must does not have past or future forms. Always use had to または will have to.
  Incorrect: I musted go yesterday.
  Correct: I had to go yesterday.

よくある質問

What is the main difference between must and have to?

The main difference is the source of the obligation. Must often expresses a personal or internal feeling of necessity (e.g., I must call my mom). Have to usually expresses an external obligation, like a rule or law (e.g., You have to pay taxes).

Can I use 'have got to' instead of 'have to'?

はい、, have got to is very common in informal spoken English and means the same as have to. 例えば、, I've got to go now とは I have to go now.

Is there a difference between 'should' and 'ought to'?

They have the same meaning and are used to give advice or recommendations. Should is much more common in everyday English. Ought to can sound a little more formal or slightly stronger, but the difference is minimal.

How do I talk about past obligations?

For past obligations, you must use had to. 例えば、, I had to work late last night. You cannot use must in the past tense.

関連レッスン

• Modal verbs
• Modal verbs
• Modal verbs
• Modal verbs