can't have been和must have been每一个的用法

情态动词+have +done 表示对过去的推测
can't have done 表示“不可能（已经）……” he can't have finnished the work，he is so lazy
他不可能已经做完了工作
must have done 只用于肯定句中，意为“一定/准是……” the road is wet，it must have rained last night。

can't have been过去做过某事对现在产生影响的否定推测 既“不可能做过某事”
must have been过去做过某事对现在产生影响的肯定推测 既“一定做过某事”
例句参照楼上

can't have been和must have been并非完成时，对过去某个时间正在发生的事、情况、状态的推测。

can't have been是表否定推测
eg:He can't have been ill,I saw him play basketball just now.
must have been是表肯定推测
eg:You must have been tired now,because you have traveled for such a long time.

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.