Needn't have done 和needn't do的区别!

问题描述:

Needn't have done 和needn't do的区别!
请告诉我具体的意思.
还有,needn't do可以用在虚拟语气中吗?
Needn't have done 一定要用在过去时态中吗?

needn't have done 是不需要做(已经做过的事)
needn't do 是不需要做(还没做的事)
不能用在虚拟
一定要的(都已经做了,当然是过去啦)