Must means "need to".
Mustn't means it is not allowed, unacceptable to do it, it is wrong to do it.
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?