"Needn't have and didn't need to
You needn't have washed the dishes. I would've put them in the dishwasher.
You didn't need to wash the dishes. I would've put them in the dishwasher.
I didn't need to prepare all that food. They phoned to say they wouldn't be coming.
I needn't have prepared all that food. They phoned to say they wouldn't be coming.
But we also use didn't need to to say that something was not necessary under circumstances where it was not done:
The sun came out so we didn't need to take any rainwear on the trip.
We had plenty of petrol in the tank so I didn't need to fill up.
We didn't need to wait for long for them. They arrived just after us.
Needn't and don't need to
You don't need to water the garden this evening. It's going to rain tonight.
You needn't water the garden this evening. It's going to rain tonight.
There's no need to water the garden this evening. It's going to rain tonight.
You don't need to shout. It's a good line. I can hear you perfectly.
You needn't shout. It's a good line. I can hear you perfectly.
There's no need to shout. It's a good line. I can hear you perfectly.
However, when we are talking about general necessity, we normally use don't need to:
You don't need to pay for medical care in National Health Service hospitals.
You don't need to be rich to get into this golf club. You just need a handicap.
Need
Need you leave straightaway? Can't you stay longer?
Need I say more? I would like you to stay.
When it is used as an ordinary verb with to before the following infinitive and with an s in the third person singular, it appears in both affirmative and negative sentences and in questions:
She's almost dehydrated. She needs a drink. She needs to drink something before she has anything to eat. She doesn't need tostay in bed, but she should have a good rest before she sets off again. ~ Do I need to stay with her? ~ Yes, I think you should"
2. Exercise.
3. Exercise.