couldn't have done和mightn't have done有什么区别

问题描述:

couldn't have done和mightn't have done有什么区别

后者语气比前者更委婉些。。。

前者表不可能做成,后者是可能做不成。

couldn't have done是 不可能做成 肯定语气
mightn't have done表 可能做不成 委婉一点