1. Ability: "Can" is used to express the ability or capability to do something.
I can swim.
She can speak five languages.
2. Possibility: "May" and "might" are used to indicate possibility or uncertainty.
It may rain tomorrow.
He might come to the party.
3. Obligation: "Must" is used to express a strong obligation or necessity.
You must finish your homework before going out.
We must arrive on time for the meeting.
4. Permission: "May" and "can" can also be used to ask for or grant permission.
May I use your phone?
Can I go to the restroom?