Modal verb: Must not/Musn't

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.

Note: 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.

From: EnglishClub.com