Needn't have and didn't need to
Both these forms are used to talk about past events, but there is sometimes a difference in use. When we say that someone needn't have done something, it means that they did it, but it was not necessary. Didn't need to is also sometimes used in this way:
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.
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