I couldn’t possibly have finished such a long book in such a short time.我想知道,为什么用 have finished ,而不用finish?

I couldn’t possibly have finished such a long book in such a short time.
我想知道,为什么用 have finished ,而不用finish?

could have do sth.表推测,这句话的意思是“我不可能在如此短的时间内完成一本书”,因为结果还不知道,所以用表推测的句型.