Learn Must and Mustn't: Expressing Rules and Importance (A1 English Grammar) — A1 English grammar lesson by Langcom

Learn Must and Mustn’t: Expressing Rules and Importance (A1 English Grammar)

ラウル・マグイニャ

Langcom 創設者 | ケンブリッジおよびSIELE試験の試験官

9月 24, 2026

A1✏️ 文法⏱️ 読了時間:3分📅 2026年9月更新
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当社では must to talk about things that are very important or necessary to do. It’s like saying ‘you have to do this’ or ‘it is a rule’. For example, You must listen to your teacher. This means it is a very important rule, and you have no choice. We also use must for strong advice or when we are sure about something. For things that are not allowed or are forbidden, we use mustn’t (which is short for must not). It means ‘you cannot do this’ or ‘it is against the rules’. For instance, You mustn’t touch the fire. This is a rule that says touching fire is dangerous and not allowed. Remember, must そして mustn’t are modal verbs. They do not change form for different subjects (like I must, he must, they must), and we always use the base form of the main verb after them.

What is ‘must’ for?

当社では must to talk about things that are very important. It shows a strong obligation or a rule.

  • Obligation or Necessity: When something is necessary to do. For example, Students must wear a uniform.
  • Strong Advice: When you think something is a very good idea. For example, You must visit the museum; it’s amazing!

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What is 'mustn't' for?

当社では mustn't (または must not) to talk about things that are not allowed. It shows a prohibition.

  • Prohibition: When something is forbidden or against the rules. For example, You mustn't take photos in the museum.
  • Danger: When something is dangerous to do. For example, Children mustn't play with knives.

How do we form sentences with 'must' and 'mustn't'?

Must そして mustn't are easy to use! They are modal verbs, so they follow simple rules.

  • Always use the base form of the verb: After must または mustn't, the next verb is always in its basic form (e.g., go, eat, study).
  • No 'to': We do not use to その後 must または mustn't.
  • No '-s' for 'he/she/it': Must never changes its form, even for he, she, 、または it.
Forms of <span data-no-translation>Must</span> そして <span data-no-translation>Mustn't</span>
Subject Positive Form Negative Form
I / You / We / They must go mustn't go
He / She / It must go mustn't go

  • I must finish my homework before dinner.
  • You must be quiet in the library.
  • He must wear a helmet when he rides his bike.
  • We mustn't talk during the exam.
  • She mustn't forget her passport for the trip.
  • They must arrive on time for the meeting.

練習

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スペイン語話者がよく犯す間違い

Many Spanish speakers make small mistakes with must そして mustn't. Here are the most common ones to avoid:

  • Using 'to' after must または mustn't:
    • You must to study.
    • You must study.
  • Adding '-s' to must for 'he', 'she', 'it':
    • He musts go.
    • He must go.

よくある質問

What is the main use of 'must'?

Must is for strong rules, obligations, or things that are very important to do. For example, You must listen to your parents.

What is the main use of 'mustn't'?

Mustn't is for things that are not allowed or forbidden. For example, You mustn't run in the corridors.

Can I say 'must not' instead of 'mustn't'?

はい、, must not is the full form, and mustn't is the short, common form. They mean the same thing.

Does 'must' change for 'he/she/it'?

No, must never changes its form. It is always must for all subjects (I, you, he, she, it, we, they).

関連レッスン

著者について:ラウル・マグイニャ

ラウル・マグイニャ氏は、認定Cambridge English試験対策センターである「Asociación Educativa Langcom」の所長であり、 あなたの国. 。語学教育および国際資格認定の分野で10年以上の経験を持ち、25カ国にわたる数千人の学生、教師、教育機関が、ケンブリッジ英語検定、IELTS、TOEFL、およびMichiganの英語資格を取得できるよう支援してきました。.

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