Must, mustn’t, needn’t

You must do something = it is necessary that you do it:

Don’t tell anybody what I said. You must keep it a secret.

You mustn’t do something = it is necessary that you do not do it ( so don’t do it ) :

It’s essential that nobody hears us. We mustn’t make any noise.

You needn’t do something = it is not necessary that you do it, you don’t need to do it:

We’ve got plenty of time. We needn’t hurry. ( = it is not necessary to hurry )

Instead of needn’t ,you can use don’t / doesn’t need to. So you can say:

We needn’t hurry. or We don’t need to hurry.

Needn’t and don’t need to are similar to don’t have to:

We’ve got plenty of time. We don’t have to hurry.

Compare needn’t ( do ) and needn’t have ( done ):

That shirt isn’t dirty. You needn’t wash it.

Why did you wash that shirt? It wasn’t dirty. You needn’t have washed it.

I didn’t need to = it wasn’t necessary for me to … ( and I knew this at the time ):

I didn’t need to get up early.

I needn’t have done something = I did something but now I know that it was not necessary:

I got up very early because I had to get ready to go away. But in fact it didn’t take me long to get ready. So, I needn’t have got up so early. I could have stayed in bed longer.