Don't worry about the homework. You ______ do it now. (Neccesity)
Don't have to / Needn't / Don't need to
He ___ be the criminal, but I'm not so sure. (Certainty)
Might / May / Could
Homer hasn't eaten yet. He _____ be hungry. (Certainty)
Must
We finished the class. You ____ go. (Permission)
Can / May / Are permitted to
