could have done有两种情况:
表示对发生过的事情的一种猜测:It's 11:00 now. I think he could have gone to bed.(现在11点了,我想他已经睡觉了。)
表示对已经发生过的事情做相反的假设,即虚拟语气:I caugh cold in the last English examination. Otherwise, I could have done better.(上次英语考试时,我感冒了,不然的话,我可能做得更好的。)
希望对你有帮助!(全手工制作的哦)