Must

Must is used to express obligation, give orders and give advice. It can only be used for present and future reference. When the past is involved, you use have to.Must is used:

to express obligation.

All pupils must bring a packed lunch tomorrow.

to give orders firmly and positively.

You must go to sleep now.

to give advice or make recommendations emphatically.

You must get one of these new smoothie-makers – they’re great!
You must see ‘Nim’s Island’ – it’s brilliant.

to speculate about the truth of something.

She must be mad!
You must be joking!
There must be some mistake.
Mr Robertson is here; it must be Tuesday.

When this sort of statement is made in the negative or interrogative, can is used instead.

Can Mary be joking? Can she really mean that?
You can’t be serious!
It can’t be true!

Must can be used in the interrogative, but many speakers prefer have to instead.

Must you go so soon?
Must I invite Helen?
Do you have to go soon?
Do I have to invite Helen?

You can use must with a negative:

to forbid someone to do something.

You must not cross when the light is red.
You must not say things like that.

to talk about an event or state that is unacceptable.

There mustn’t be any mistakes in your letter.
The whale must not become extinct.

Note that to express the fact that you are not obliged to do something, you use do not have to.Compare:

You must not come in here.
You don’t have to come in here (if you don’t want to).

It is necessary to change must to have to when changing sentences from direct to reported speech.

‘I must fill out those forms this evening,’ said Ian.
Ian said that he had to fill out some forms.

must
The contracted negative form is:	mustn’t.

You mustn’t worry so much.