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English Club : Learn English : Grammar : Verbs : Modal Verbs : Have to / Must

Must not, Mustn't (prohibition)

We use must not to say that something is not permitted or allowed, for example:

  • Passengers must not talk to the driver.

Structure of Must not

Must is an auxiliary verb. It is followed by a main verb. The structure for must not is:

subject + must not + main verb

The main verb is the base verb (infinitive without "to").

Must not is often contracted to mustn't.

Look at these examples:

subject auxiliary must + not main verb
I mustn't forget my keys.
You mustn't disturb him.
Students must not be late.

NB: like all auxiliary verbs, must CANNOT be followed by "to". So, we say:

  • You mustn't arrive late. (not You mustn't to arrive late.)

Use of Must not

Must not expresses prohibition - something that is not permitted, not allowed. The prohibition can be subjective (the speaker's opinion) or objective (a real law or rule). Look at these examples:

  • I mustn't eat so much sugar. (subjective)
  • You mustn't watch so much television. (subjective)
  • Students must not leave bicycles here. (objective)
  • Policemen must not drink on duty. (objective)

We can use must not to talk about the present or the future:

  • Visitors must not smoke. (present)
  • I mustn't forget Tara's birthday. (future)

We cannot use must not to talk about the past. We use other structures to talk about the past, for example:

  • We were not allowed to enter.
  • I couldn't park outside the shop.

MUST NOT for Future Prohibition Games

Now check your understanding »

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