因为这是表示推测的,推测的内容是过去的——即今早,
所以,要由意思,对过去时进行反问.
这种对过去情况表示推测的句子,有很多.
He must have left yesterday, didn't he?
他昨天一定走了,是吗?
It must have snowed yesterday, didn't it?
昨天一定下过雪,是吗?
You must have arrived yesterday, didn't you?
你一定是昨天到的,对吧?