只要记住这:
must have been ...那时一定是can't have been...那时不可能是
再好好理解这几人句子,相信你就理解啦.
He can't have been ill.He must have been tired.
She can't have been forty.She must have been fifty.
He can't have been the youngest.He must have been the oldest.
He can't have been listening carefully.He must have been dreaming.