You use must to indicate that you think it is very important or necessary for something to happen. You use must not or mustn't to indicate that you think it is very important or necessary for something not to happen.

You use must to indicate that it is necessary for something to happen, usually because of a rule or law.

You use must to indicate that you are fairly sure that something is the case.

You use must, or must have with a past participle, to indicate that you believe that something is the case, because of the available evidence.

If you say that one thing must have happened in order for something else to happen, you mean that it is necessary for the first thing to have happened before the second thing can happen.

You use must to express your intention to do something.

You use must to make suggestions or invitations very forcefully.

You use must in remarks and comments where you are expressing sympathy.

You use must in conversation in expressions such as 'I must say' and 'I must admit' in order to emphasize a point that you are making.

You use must in expressions such as 'it must be noted' and 'it must be remembered' in order to draw the reader's or listener's attention to what you are about to say.

You use must in questions to express your anger or irritation about something that someone has done, usually because you do not understand their behaviour.

You use must in exclamations to express surprise or shock.

If you refer to something as a must, you mean that it is absolutely necessary.

You say 'if you must' when you know that you cannot stop someone doing something that you think is wrong or stupid.

You say 'if you must know' when you tell someone something that you did not want them to know and you want to suggest that you think they were wrong to ask you about it.