“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. Without your help, I wouldn't have achieved so much. 没有你的帮助,我是不会取得如此大的成绩.
“could+have+done”是虚拟语气,表示对过去事情的假设,意思是本来能够做某事而没有做.
He could have passed the exam, but he was too careless. 本来他能够通过考试,但是他太粗心.
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should have done表示应该做的事没做