Must is usually followed by an infinitive without ‘to’: You must stop at the red light. Sometimes must is used without a following infinitive: We will act alone if we must.
Must does not change its form, so the third person singular form does not end in ‘-s’: She must be asleep.
Questions are formed without ‘do’: Must we wear our uniforms? Must can be used in question tags: We mustn’t be late, must we?
The negative form must not is used for saying that something should not be done. This is often shortened in conversation or informal writing to mustn’t: You mustn’t use the office phone for private calls.
For saying that it is not necessary to do something not have to, not need to, or needn’t is used: Children don’t have to pay to go in.You don’t need to take the test.
There is no infinitive form of must, so to have to is used instead: I didn’t expect to have to do my own cleaning.
There is no past tense, but had to is used for saying that something was necessary in the past: We had to show our passports. Must have followed by a past participle can be used for saying that you believe something happened in the past: They must have missed the train. When indirect speech is introduced by a verb in the past tense, must can be used as a past tense: She said that I must come immediately.
There is no future tense, but must can be used for saying that someone should do something in the future: You must be here at nine o’clock tomorrow morning. For saying that something will be necessary in the future will have to can be used: Graham will have to sleep on the sofa.