Didn't need to do还有 needn't have done的区别

问题描述:

Didn't need to do还有 needn't have done的区别
如题
我怕学生会听不懂,最好是英文版的。

didn't need to表示过去不需要做,但是也许做了,也许没做,不确定;needn't have done表示过去不需要做的事已经做了