填isn't he.
含有推测语气的句子按去除推测的实际情况做反义疑问句.如：
You must be joking,aren't you?
He can't be at home now,is he?
They might have finished their work already,haven't they?
They might have finished their work yesterday,didn't they?否定部分是什么由构成的？You must be joking去除推测相当于You are joking所以后面用aren't you?He can't be at home now去除推测相当于He isn't at home所以后面用is he?They might have finished their work already去除推测相当于They have finished their work所以后面用haven't they?They might have finished their work yesterday去除推测相当于They finished their work yesterday所以后面用didn't they?去除推测后做反义问句和平常的一样了。