Must

Something that is mandatory or required.

"If you'll be out all day, a map is a must."

(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."

(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."

(modal auxiliary, defective) said about something that is very likely, probable, or certain to be true

"The children must be asleep by now."