The site https://englishlive.ef.com is excellent. https://englishlive.ef.com/blog/language-lab/english-grammar-use-must-mustnt/

Thank you (:

Not often. In casual talk, people will usually substitute "have to", as in, "You have to eat or you will stay skinny." Sometimes English speakers will use "must" to emphasize their is no option or ambiguity, as in a parent's "you must eat before you go outside to play." However, it would be more natural for a parent to say, "You can't go outside to play until you eat." "Will" is often used instead of "must" when someone wants to be very forceful and unambiguous as in, "You will eat your broccoli." "If you must know," is snarky phrase used to answer an inappropriate question.

He must have been mistaken. You must not worry about silly things. They must have left without him. She must do her homework before going outside.

"must" is imperative. Use must when something HAS to get done. "I must feed my dog, or she'll be hungry" or "I must go to work on time or I'll get fired"