Have to shows that something is necessary. Don’t have to means that something is not necessary.
Have to can be used in the past, present, or future.
- You have to stay calm. (present)
- You’ll have to give your address when you call. (future)
- We had to call 911. (past)

Must also shows that something is necessary. It’s stronger than have to. You won’t hear must so much in spoken English, but you’ll often see it written. The negative is must not, which expresses that something is not allowed. It can be used in the present or future sense only. Its form does not change.
- You must stay on the line. (present)
- You must call later. (future)
- You must not hang up if you call 911 by mistake.