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.
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.