Special Uses of Must And Cannot

Must can be used to express the conclusion that something is certain or highly probable.

To express certainty in questions, we usually use can. Note that must is not used in questions with this meaning.

Cannot can be used in negative clauses to say that something is certainly not the case.

Need not can be used to say that something is not necessarily so. Must not is not used to express this meaning.

The structure must have + past participle is used to express conclusions about the past.