Modals

Basic rule

"Can", "may" and "must" are used in the present tense. But "may" is only used in questions. In the other tenses you usually use "(to) be able to", "(to) be allowed to" and "(to) have to".

Deeper explanation

Present	Past	Future
Can	was/were able to	will be able to
May	was/were allowed to	will be allowed to
Must	had to	will have to

If you use these forms in the past or in the future, you'd have to change the the tense of the verb (like you see in the table). In the past you usually use simple past and in the future the will-future. These forms can also be used in the present tense.

Examples

Present

Can: I can go camping if I want to.
May: