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must not for prohibition

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

Structure of must not

Must is an auxiliary verb. It is followed by a main verb.

The basic structure for must not is:

subject + must not + main verb

The main verb is the base verb.

We often contract must not 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:

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:

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

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