Must

Must" is most commonly used to express certainty. It can also be used to express necessity or strong recommendation, although native speakers prefer the more flexible form "have to." "Must not" can be used to prohibit actions, but this sounds very severe; speakers prefer to use softer modal verbs such as "should not" or "ought not" to dissuade rather than prohibit.



Examples:
This must be the right address! certainty
Students must pass an entrance examination to study at this school. necessity
You must take some medicine for that cough. strong recommendation
Jenny, you must not play in the street! prohibition