Definition must in American English

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 tag questions: We must be almost there, mustn’t 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: Jason will have to sleep on the sofa.