有一读者问：有一道选择题：
(1)The ground is wet. It must have rained yesterday, _________it?
A. mustn't B. haven't
C. must D. it
答案是A。而另一本教学参考书上则有一个例句是：
(2)They must have seen the film yesterday, didn't they?
哪一个对？
有的语法书上是这样说的：当 must 意指“必须”时，反意疑问句仍应用 must 或 mustn't，如：
(3)We must study hard, mustn't we ?
(4)We must obey orders, mustn't we ?
但当 must 表推测时，反意疑问句则不应用 must，而应用动词或助动词，如：
(5)You must be a writer, aren't you ?
(6)You must have lived here for a long time, haven't you ?
(7)You must have seen him yesterday, didn't you ?
按照这条规则，上述句（2）是对的，句（1）就错了。然而，这条语法规则并不为人人所遵守，must 表推测时其反意疑问句用 must 的情况也不罕见，如：
(8)He must have left, mustn't he ?
(9)Some of them must have got out, mustn't they ?
这样说来，上述句（1）就不能算错了。
