“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 疑问句:①Could you have done the exercises? 你可能做完练习吗? 否定句②They couldn’t have finished the work so soon. 他们不可能这么快就做完了。
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