Modal Verbs
Auxiliary verbs that can not function as a main verb, after a modal verb the verb must come in its base form.
Must
We use must when we feel sure about something because there is some kind of evidence.
He must be rich
Can
We use can when we feel sure something is true.
They can be poor
Others
Could
Peter could be at the library.
May
They may win the match.
Might
She might go to the party tomorrow if you ask her.