Mustn’t or Needn’t ?

 We use mustn’t to say that something is a bad idea.

• You mustn't phone me at work. We aren't allowed personal calls.
• Children mustn't play with fire.
• They mustn’t see us talking or they’ll suspect something.

We use needn’t when something is not necessary.(but you can if you like)

• We've got plenty of time. We needn't hurry. (=it is not necessary to hurry)
• You needn’t pay the money now. You can give it later.
• I needn’t take an umbrella. It isn’t raining.

We can use don’t need to and don’t have to when something is not necessary.

• Today the museum is free. You needn’t / don’t have to/ don’t need to buy a ticket.

needn't have (done)

If we say that somebody “needn’t have done” something, we mean that the action is done, but it was unnecessary and even a waste of time.

• Sandra worked all the weekend to finish her assignment on time, but actually she needn’t have done so.
• They needn't have reserved a table. =They reserved a table, but the restaurant was almost empty.This was not necessary.

didn't need to (do)

We use didn’t need to when something was not necessary.

• You didn't need to buy a present for me, but I appreciate it. (It was not necessary to do it. lt doesn't matter whether he did it or not.)
• He didn't need to get up early, so he didn't.
• We had plenty of petrol in the tank so I didn't need to fill up.