1 We use modal verbs in a situation where we need some level of deduction which means we say how
sure we are about something.
2 Must We use 'must' when we feel sure that something is true because we have some information
about the situation, we have strong evidence. She must live close to where she works because she
walks to work. (the speaker doesn’t know where but is sure it is not far away) You’ve been working in
the garden all day. You must be tired. Being a sky diving instructor must be very exciting.
3 Can't We use 'can't' to show that we think something is not true. It can’t be a mechanical problem.
I've just taken the car to a mechanic. They can't be very happy about working during the night.
(can't+infinitive without 'to')
4 Might, may, could We use 'might', 'may' and 'could' when we are not sure about something. I don't
like the sound of the engine. It could be a mechanical problem. We might need to work through the
night if we want to get this ready on time. He may not like that shirt. I don’t think it's his style.
(might/may/could+infinitive without 'to')
5 We use might, may or could to say that we think something is possible but we’re not sure.