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may have V-ed 结构︰主词＋may／might have＋过去分词 说明︰此句型意为很可能曾经。表示对过去之事做不太肯定的推测。而may／might＋原形动词是对现在或未来事物做推测。might have 所表示的可能性要比 may have 低一些。 He may have overslept this morning. 今天早上他或许睡过头了。 The tea is hot； he may have been here before. 茶还热着，不久前他很可能在这里。 He may have known the truth； otherwise he wouldnt be so angry now. 他当时很可能知道事情的真相，否则他不会这么生气。 The boy might have known the truth， but I am not quite sure. 这男孩当时也许知道真相，但我不很确定。 cannot have V-ed 结构︰主词＋cannot have＋过去分词 说明︰此句型意为不可能曾经。是对过去之事做否定的推论，此时一定要用 cannot have 或 cannot，不能使用 must not have 或 must not.而cannot＋原形动词是对目前或将来之事做否定的推论。 She cannot have written the letter herself. 这封信不可能是她自己写的。 He is honest， so he cannot have stolen my money.他是个老实人，所以他不可能偷了我的钱。 He cannot have caught a cold， because I saw him dancing with Mary the same day. | 
