Study the example sentences below:
The train leaves at 5.45. We must hurry!
We must be quiet in the library. You mustn’t use a loud voice.
I can go by myself, you needn’t come.
You must do something = it is necessary that you do it.
You mustn’t do something = it is necessary that you don’t do it.
You needn’t do something / don’t need to do something = you don’t need to do it but you can if you want.