Must means need to do something.
Mustn't means it is not allowed, unacceptable, wrong to do something.
I must hurry or I'll be late.
You must watch the second season! You'll like it.
We must fix the tap. It's leaking.
Son, you mustn't behave like this. It is impolite.
Remember: must + infinitive
|Positive and negative|
I must go now.
They mustn't forget about it.
Why must you do that?