We use must when:
- We are sure about something, something is reasonably expected (affirmative);
- There is strong obligation (affirmative, interrogative);
- Something is necessary (affirmative, interrogative);
- Prohibition (negative).
- It must be her dog. The collar says her name.
- This house must be in a good state! They renovated it last year.
- Must they be rich? They live in a luxury flat in the city centre.
- I must search for a new house, my contract ends soon.
- She must clean the kids’ clothes, they got very dirty at the park.
- Must we be on time?
- I must buy a new bunch of flowers.
- He must search for a student flat.
- Must they chop the olive trees down?
- You must not give your address on online forums!
- He must not surf the internet while working!
- You mustn’t smoke in this restaurant.