Use modals of deduction to say how sure you are about something.
- could / might / may – for things / actions that are possible
Example: He could be our new teacher.
That girl might be his new girlfriend.
The man in black may be his grandfather.
- must – for things / actions that are almost certain
Example: She must be his sister.
- can’t – for things / actions that are impossible
Example: That lady can’t be our new principal. I saw her working at a grocery store last week.