主要由两部分构成：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。此外，连接词“whether”也适用。主要具有以下几种功能：
⑴当主语
● When to hold the meeting has not yet been decided.
● Where to live is a problem.
⑵当宾语
常接在forget, find out, discuss, decide, tell, teach, know, learn, wonder, remember等动词之后作宾语如：
● We must know what to say at a meeting.
● He could not tell whom to trust.
● Do you know how to play bridge?
⑶当表语
● The problem is where to find the financial aid.
● The question is who to elect.
⑷当名词同位语
● Tom had no idea which book to read first.
● The question whether to confess troubled the girl.
⑸用于宾语补足语，双重宾语
某些动词后可接“sb.+ 疑问词 + 不定式”构成的双重宾语，这类动词常用的有advise, ask, show, teach, tell等
● I can tell you where to get this book.
● I ask her how to learn English.
● He will advise you what to do.
● Will you show me how to use the machine?
(6) 作介词宾语
某些介词后面可接'疑问词＋不定式'作宾语，这类介词常用的有 about, as, from, in, of, on, with等。例如：
He has no idea of how to answer this question. 他不知道怎样回答这个问题。
Della had only $1.87 with which to buy Jim a present. 德拉仅有$1.87为来吉姆买一件礼物。(which是关系代词)
You have a number of topics from which to choose. 你有很多题目可以选择。(which是关系代词)
如： I don’t know what I’ll do next. = I don’t know what to do next.
The doctor told him when he should take the medicine. = The doctor told him when to take the medicine.
同样，因为“特殊疑问词＋动词不定式”这种结构相当于一个名词性从句，所以可用同等成分的从句代替，从而把简单句转换为复合句。改写时只需在疑问词后加上适当的主语，并把不定式改为适当的形式的谓语即可。如：
Which to choose is important. = Which we should choose is important.
The question is where to go. = The question is where we should go.
比如：
What to do? 怎么办？
How to get rid of the trouble? 如何摆脱困境？
按照英语语法的习惯，疑问副词why 后不直接跟动词不定式，如：
误：I don’t know why to choose that dictionary.
正：I don’t know why you must choose that dictionary.
但它可以接不带to的动词不定式，即动词原形，构成why do sth./ why not do sth. 肯定形式是表示某动作是不必要的或没有意义的，而否定形式是表示向某人提建议和表达看法，相当于祈使句。
Today is Sunday. Why not go out to relax yourself?
Why pay more at other shops? We have the best value.
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