CAN, HAVE TO, MUST

Use CAN to talk about things that are possible/allowed. For example:

You can send me the letter by email.

(It's possible to send it by email)

Use CAN'T to talk about things that are NOT possible/allowed.

You can't talk in the exam.

(You are not allowed to talk in the exam.)

Use them to talk about rules or things that are necessary. For example:

We have to study for the exam. (It's necessary.)

She has to do her homework. (It's necessary)

You must clean your area before you leave. (It's a rule.)

You mustn't copy in the exam. (It's a rule.)

We use it to talk about something that is not necessary but is possible or allowed. For example:

You don't have to do extra work. (It's not necessary to do extra work, but if you want to you can do it).

She doesn't have to buy the book. (It's not necessary to buy the book but you can do it.)