We use must to express necessity, opinion, obligation, suggestion, insistence, and question.
- I haven’t eaten a pear for a long while. I must pluck one from my neighbour’s tree. (Necessity)
- She has been talking for so long her mouth must be very dry now. (Opinion)
- You must obey the rules and not bring your pet here. (Obligation)
- You must smile more often. (Suggestion)
- We have a pond, so we must keep some ducks. (Insistence)
- Must you visit the toilet so many times? (Question)