It must rain last night.isn't it?这整个句子都是错的.用isn't it来反问,更是错的.
(The playground is wet this morning)早上操场还是湿的,由此推测昨晚肯定下雨了.
对过去情况肯定的推测,应用must have done sth.,其反意问句是用didn't 来反问.
因此It must rain last night.isn't it?应改为:
It must have rained last night,didn't it?
昨晚肯定下雨了,对吗?