由两部分构成：疑问词和动词不定式（wh-+to do）。疑问词部分包括疑问代词who, whom, what, which和whose以及疑问副词when, where和how。作用：按所作的句子成分来说，在句子中一般可以作主语，宾语和表语。
1.作主语Where tomeet is a problem.
2.作宾语
（1）作动词的宾语
I can’t decide when to start.我不能决定何时动身。
（ 2）作介词的宾语
I am thinking about how to answer the question.
3.作宾语补足语
He taught me how to sing the song.
4.作表语
The first step is which to take.