1. Modals of possibility and certainty
MAY / MAY NOT / MIGHT / MIGHT NOT / COULD  present or future possibility
We might have nothing in common.
They could be on the train now.
CAN’T  when we believe or guess that something is impossible
He can’t be boring – he is in a band.

2. Modals of possibility and certainty
MUST  when we are certain something is true
He must be a nice person if he is Jeff’s friend.
MUST / CAN’T  when we make logical deductions
Linda can’t be at home because she didn’t answer the phone.