1. To provide something's necessity:We must live our life without any fear.
The students must carry their ID cards during exams.
2. To express a strong action:
He must be his brother-in-law.
It must be her last attempt.
3. To express prohibition:
Anyone must not leave their vehicles everywhere in the public area.
You must not discuss loudly in class.
4. To comment, provide reviews or feedbacks on something:
We must be working hard to clear this exam.
5. In exclamations:
You must be joking!
6. To emphasize something:
I must tell you that you are a great person.