Mastering Must and Mustn't: Expressing Obligation and Prohibition (A2 English Grammar) — A2 English grammar lesson by Langcom

Mastering Must and Mustn’t: Expressing Obligation and Prohibition (A2 English Grammar)

劳尔-马吉尼亚

Langcom 创始人 | 剑桥考试和SIELE考试考官

8 月 5, 2026

A2✏️ 语法⏱️ 阅读时间 5 分钟
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Mustmustn’t are modal verbs we use to talk about rules, obligations, and things that are very important to do or not to do. 我们使用 must when something is necessary or when we give strong advice. For example, You must wear a seatbelt in the car. This shows a rule or a strong necessity. On the other hand, we use mustn’t (which is short for must not) to say that something is forbidden or not allowed. For instance, You mustn’t smoke here means it is against the rules. These verbs help us express clear instructions and prohibitions in English, making your communication precise and easy to understand.

What Does ‘Must’ Mean?

我们使用 must to express a strong obligation or something that is necessary. It often comes from the speaker’s own feeling or a general rule.

  • For rules and obligations: This is for things you have no choice about.
  • Students must arrive on time for class. (It’s a rule.)
  • I must finish this report by Friday. (It’s a personal obligation.)
  • For strong advice or recommendations: When you think something is very important for someone to do.
  • You must try this cake; it’s delicious! (Strong recommendation.)
  • You must study hard for your exam. (Strong advice.)

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How Do We Use 'Mustn't'?

Mustn't is the short form of must not. We use it to say that something is forbidden or not allowed.

  • For prohibitions: When something is against the rules or dangerous.
  • You mustn't touch the exhibits in the museum. (It's forbidden.)
  • We mustn't make a lot of noise in the library. (It's not allowed.)
  • For strong warnings: To tell someone not to do something because it's bad or risky.
  • You mustn't forget your passport. (It's very important.)

What is the Grammar Pattern for 'Must' and 'Mustn't'?

The structure for using mustmustn't is very simple.

  • 我们使用 mustmustn't followed by the base form of the verb (the infinitive without 'to').
  • Subject + must/mustn't + base verb
  • I must go.
  • You mustn't run.
  • Mustmustn't do not change form for different subjects (I, you, he, she, it, we, they).
  • He must work. (NOT He musts work.)
  • They mustn't talk. (NOT They mustn'ts talk.)

Must vs. Have to (and Had to)

虽然 must expresses obligation, have to is also used for obligation, especially for external rules or duties. For A2 learners, it's good to know the difference.

  • Must: Often personal obligation or strong advice from the speaker.
  • I must call my mother. (I feel it's important.)
  • Have to: Often an external rule or requirement.
  • I have to wear a uniform at work. (It's a company rule.)
  • We usually use have to for questions and past obligations.
  • Questions: Do you have to work tomorrow? (NOT Must you work tomorrow?)
  • Past: 使用 had to for past obligations.
  • Yesterday, I had to wake up early. (NOT Yesterday, I must wake up early.)
Summary of Must and Mustn't
Verb Meaning 示例
Must Obligation / Strong Advice You must be quiet in the hospital.
Mustn't Prohibition / Not Allowed You mustn't eat in the classroom.

示例

  • You must listen carefully to the instructions.
  • We mustn't forget to buy milk.
  • She must study for her English exam.
  • Visitors mustn't feed the animals.
  • I must remember to send that email.
  • They mustn't be late for the meeting.

练习

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西班牙语母语者常犯的错误

讲西班牙语的人有时会犯以下错误:

  • Using 'to' after 'must' or 'mustn't': In Spanish, you might use 'tener que + infinitivo'. But in English, must is followed directly by the base verb.
  • INCORRECT: You must to study.
  • CORRECT: You must study.
  • INCORRECT: We mustn't to talk.
  • CORRECT: We mustn't talk.
  • 令人困惑 mustn'tdon't have to: They have different meanings.
  • Mustn't means 'it is forbidden' or 'not allowed'.
  • You mustn't park here. (It's illegal.)
  • Don't have to means 'it is not necessary'.
  • You don't have to park here. (You can, but it's not required.)

常见问题

What is the main difference between 'must' and 'mustn't'?

Must expresses an obligation or strong necessity, like You must wear a helmet when cycling. Mustn't expresses a prohibition or something that is not allowed, like You mustn't use your phone during the exam.

Can I use 'must' for past obligations?

No, must is only for present or future obligations. For past obligations, we use had to. 例如,, Yesterday, I had to work late.

How is 'must' used in the A2 Key (KET) exam?

In the A2 Key exam, you might see mustmustn't in reading tasks, like signs or short messages, where you need to understand rules or instructions. For example, a sign might say Visitors must report to reception, and you need to understand this obligation.

Does 'must' change its form for 'he' or 'she'?

No, must is a modal verb, so it never changes its form. You always say I must, you must, he must, she must, etc., followed by the base form of the verb.

关于作者:劳尔·马吉尼亚

劳尔·马吉尼亚是Langcom教育协会的主任,该协会是位于 巴西. 凭借在语言教育和国际认证领域十余年的经验,他已帮助来自25个国家的数千名学生、教师和教育机构考取了剑桥英语证书以及IELTS、TOEFL和Michigan英语证书。.

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