Study the differences between must and have to.
1. Must can be replaced by have to with little difference in meaning:
You have to study. (= you must study)
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2. Have to is a more informal while Must is mostly used in written orders or instructions.
Boss: You must complete this work by Friday
Worker:  We have to complete this essay by Friday.
3. When we are mentioning someone else's obligations, we use have to.
John has to quit working in this company.

4. For questions it is more common to use have to instead of must (which sounds very formal):
When do you have to finish the report?
5. The past tense of must is had to:
I had to pay my bill yesterday.
Mustn't  and Don't have to:

Mustn't is a negative obligation (= it is important that you do NOT do something) while don't have to is an absence of obligation.
Mustn't = it is prohibited; it is not allowed
Don't have to = no obligation; you are not required to do something, especially if you don't want to.
You must not drink this water. (= it is forbidden to drink that; it is not allowed)
You don't have to drink this juice. (= you don't need to drink that but you can if you want)