We use both must and have to to express obligation or necessity, but there is sometimes a difference between them:

Asking for permission

 We use  can, could, may and  might to ask for permission, depending on the situation.

1.  Can is the commonest and most informal: Can I borrow your umbrella (please)?
2.  Could is more polite than can: Could I borrow your umbrella (please)?
3.  May is more formal, more respectful than can and could: May I borrow your umbrella (please)?
4.  Might is the most polite but the least common: Might I borrow your umbrella (please)?
5.  We can add possibly and use the expressions like Do you think and  I wonder if to make requests even more polite.

We use can to say that someone has the ability or opportunity to do something. The negative of can is cannot (contraction: can't).