同学你好，该知识点来自沪江网校《Hitalk流利生活口语1v6精品超小班150次卡 【现金奖励班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好，I should /shouldn't have是虚拟语气，对过去的虚拟，“如果我过去怎样/没怎样 ”
例句：I should have gone this morning but I was feeling a bit ill 今天早晨我本应该走的，但是我觉得有点不舒服。

If I had....和If I hadn't... ，如果后面接done，也是对过去的虚拟，例：If I had known it would come to this, I would have acted differently.
早知今日，何必当初？
如果是had就是谓语，比如if I had nine lives 如果我有九条命，是if引导的虚拟语气，是对现在的虚拟和假设，所以谓语动词用过去式had

.I could have....我本能做某事
I would have 我本要做某事
这也都是虚拟语气，对过去事实的假设，
例句：“would+have+done”虚拟语气，表示对过去事情的假设，意思是“本来会做”。
1. I would have told you all about the boy's story, but you didn't ask me. 我本来会告诉你这个小男孩的故事，但是你没有问我。
2. I could have told the story better if so many people had not broken in.
假如没有那么多人插嘴，我的故事还能讲得更好些。

希望可以帮到你哦～