(modal auxiliary, defective) To do with certainty; indicates that the speaker is certain that the subject will have executed the predicate.
If it has rained all day, it must be very wet outside.
2
(modal auxiliary, defective) To do as a requirement; indicates that the sentence subject is required as an imperative or directive to execute the sentence predicate, with failure to do so resulting in a negative consequence.
This door handle must be rotated fully.
3
(modal auxiliary, defective) said about something that is very likely, probable, or certain to be true