Modals of deduction: may and might
These modals are used when we think that something is possible. It’s very similar to saying ‘perhaps’ A:Where are they going for their holidays?
B: I don’t know. They may/might go to Scotland because they want to see the Highlands. (meaning perhaps)
Modal of deduction: must
We use must when we are very sure about something.
A: Shall I call Linda at the office?
B: Yes, it’s already 10:00 so she must be there right now.
Modal of deduction: can’t
We use can’t when we are sure that something is not possible or not true.
A: Look! There are lights in the house. Linda must be in.
B: She can’t be in the house! We talked to her twenty minutes ago on her phone and she was still at the office.