有一读者问这样一道选择题：
(1)I don't know_________to do next.
A. what B. what I C. what am I
老师说应选A。这是为什么？ 应如何翻译？
这里之所以应选 what是因为只有what to do 合乎语法。“what+带to 的不定式”是一种特殊的不定式短语，它有“应该”的含义，what to do相当于what one should do，可译作“该做什么”。
其他疑问词如who, which, when, where, how 等也可以后接带to的不定式，而且也都有“应该”的含义，实例如：
(2)I don't know who to speak to .
(3)The book says one thing and you say another. I don't know which to believe .
(4)I don't know when to start .
(5)I don't know where to go .
(6)I don't know how to call him.
