must / have to — Have To

Use must and have to to express obligation or necessity. Their meanings are similar, but there are important differences in nuance and usage.

Basic Rules

must + base form of the verb expresses “must do ~” (strong obligation) or “must be ~” (strong certainty). It is natural when the speaker personally feels that something is necessary.
have to + base form of the verb expresses “must do ~” because of an external necessity, rule, or situation. It is more objective and neutral than must. With a third-person singular subject, it becomes has to.
An important point is the negative form: must not means “must not do ~” (prohibition), while don't have to means “do not need to do ~” (no necessity). Their meanings are very different, so never confuse them!
must cannot be used in the past or future, so use had to / will have to.

Points to Note

① must not and don't have to have opposite meanings: “prohibited” and “unnecessary.” “You must not eat this.” and “You don't have to eat this.” are completely different.
② must has no past, future, or perfect form. Use had to for “had to do ~ in the past” and will have to for “will have to do ~ in the future”: “I had to study hard last night.” and “I will have to apologize.”
③ have to is treated as an ordinary verb, so use do / does / did in negative sentences and questions: “Does he have to go?” and “I didn't have to wait long.” Since must is a modal verb, do is not used (× He doesn't must).

Practice

1. Write “You must go to bed early” using must.
2. Write “He does not have to go to school tomorrow” in English.
3. Write “You must not take pictures here” using must not.