결론적으로 둘의 차이는
현재는 뜻은 같지만 needn't가 좀 더 former 한 것이고,
과거는 didn't need to 는 필요로 하지 않아서 행위를 하지 않았고
needn't have done의 경우는 필요로 하지 않은데 한 경우이다.
아래의 예로 보면
이미 시험을 통과했기 때문에 걱정을 할 필요가 없다.
오늘이 공휴일인걸 몰라서 회사에 가지 않아도 되는데 갔다.
이런 차이다.
Hi, everybody. I'm Sam, and in this lesson I'm going to talk about need.
Do you know the difference between needn’t and not need to?
Look at these two sentences in the present – is the meaning the same or different?
You don't need to worry about the exam. You've studied lots.
You needn't worry about the exam. You've studied lots.
They're the same right? Maybe needn't is a little bit more formal,
but the meaning is that there is no necessity for you to worry.
Now look at these two sentences in the past:
He didn't need to worry about the exam. He had already passed the course.
He needn't have worried about the exam. It was really easy.
These are different! In the first sentence, he had already passed the course,
so he knew he didn't need to worry.
In the second sentence, he didn't know the exam was going to be easy,
so he was worried, but he needn't have been.
-Needn't have done vs Not need to
We can use needn't and not need to in the present and
both mean something is not necessary.
The meaning is the same, but needn't is slightly more formal.
I needn't go to work tomorrow as it's a holiday.
I don't need to go to work tomorrow as it's a holiday.
When we use needn't and not need to in the past, the meaning is different.
Didn't need to do something means that
we did not do something and it wasn't necessary.
Needn't have done something means that we did something, but it wasn't necessary.
I didn't need to go to work today, so I stayed in bed.
I needn't have gone to work today. I didn't realise it was a holiday.
What a mistake!