Modals of deduction

We use must when we feel sure about something because there is some kind of evidence.

e.g.   She must study very hard because she gets really good marks.

We use can't when we are sure something is not true.

e.g. You have been sleeping all day. You can't be tired.

We use may, might and could when are not sure about something, but we think it is possible.

e.g. She might go to the party tomorrow if you ask her. 

e.g. Peter could be at the library. He said he was going to go one of these days.

e.g. They may win the match. They are quite good.

***They all have the same meaning, but may is more formal than might and could.