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.