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. — the requirement is a directive.
You say `if you must' when you know that you cannot stop someone doing something that you think is wrong or stupid. If you must be in the sunlight, use the strongest filter cream you can get `Could I have a word?' --- `Oh dear, if you must.'
feelings You use must in questions to express your anger or irritation about something that someone has done, usually because you do not understand their behaviour. Why must she interrupt? Must you always run when the pressure gets too much?