题目内容：

They must have done their homework yesterday,___?
为什么是didn't they?
我觉得应该是haven't they啊?

优质解答

must have done ：对过去事实的推测,不是现在完成（后者不能与明确的过去时间连用）.反义问句是问“昨天他们是否做了家庭作业”,当然用didn't they.
比较：
They have done their homework,haven't they?
They did their homework yesteday,didn't they?