why not,why didn't,why couldn't的区别

why not,why didn't,why couldn't的区别
why not,why didn't,why couldn't的区别

why not,why didn't,why couldn't的区别
why not 为什么不
why didn't 为什么没有
why coudn't 为什么不能
1.Why not
A:I don't want to go.我不想去
B:Why not?为什么（不想去）呢?
2.Why didn't
Why didn't you go yesterday?你昨天为什么没有去呢?
3.Why couldn't
Why couldn't I go with you yesterday?
我昨天为什么不能跟你一起去呢?

事实上是没区别的 所谓的区别都是国人自己研究出来的 当你在国外用的时候没有任何区别