Must is a Modal Auxiliary Verb.
(The other modals in English are may, might, can, could, will, would, shall, and should.
What follows applies to all of them, not just must.)
All modals have two different types of meaning:,
- one meaning, called the Epistemic sense of the modal, deals with possibility and probability
e.g, using may and must as examples:
This may be the place (may = logically possible)
This may not be the place (may not = possibly not)
This must be the place (must = logically necessary)
This must not be the place (must not = logically impossible)
- another meaning, called the Deontic sense of the modal, deals with permission and obligation
You may attend the prom. (may = permitted to)
You may not attend the prom. (may not = forbidden to)
You must attend the prom. (must = obliged to)
You must not attend the prom. (must not = forbidden to)
As you can see, negatives have very complex patterns with modals,
but modals are plenty complex all by themselves.
Deciding which one is intended is a matter of interpretation and context, and certainly active predicates with human agent subjects, like He must do it, are more likely to be deontic, while stative predicates, like He must be late, are more likely to be epistemic.
However, this is not true in all cases; consider for example
- He must do this every morning. (must = logically necessary)
- He must be at work by 07:40. (must = obliged to)