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What Does ‘Must’ Mean?

We use 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.)

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 must and mustn't is very simple.

• We use must or mustn'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.
• Must and mustn'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)

While 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: Use 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	Example
Must	Obligation / Strong Advice	You must be quiet in the hospital.
Mustn't	Prohibition / Not Allowed	You mustn't eat in the classroom.

Examples

• 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.

Practice

Common mistakes for Spanish speakers

Spanish speakers sometimes make these mistakes:

• 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.
• Confusing mustn't with don'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.)

Frequently asked questions

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. For example, Yesterday, I had to work late.

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

In the A2 Key exam, you might see must and mustn'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.