“疑问词+不定式动词”其主要功用有下列五种：
⑴当主语，如：
● When to hold the meeting has not yet been decided.
● Where to live is a problem.
● How to cope with the rising cost of living becomes a daily discussion topic.
⑵当宾语，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当补足语，如：
● The problem is where to find the financial aid.
● The question is who to elect.
⑷当名词同位语，如：
● Tom had no idea which book to read first.
● Do you have a rough impression how to do it?
⑸当宾语补足语，如：
● Jim is not sure whose to choose.
● Mary and John are not certain whether to get married or not.
适用于“疑问词+不定式动词”的动词包括：“know, see, decide, tell, ask, consider, discover, explain, forget, guess, hear, imagine, inquire, learn, remember, think, wonder, understand”等。
有点值得特别注意的是：当这结构当宾语时，它的作用等于名词分句，例如：
● I could not decide which dictonary to buy. / I could not decide which dictionary I should buy.
● Jack did not know where to find such a good teacher./ Jack did not know where he could find such a good teacher.
有些动词，如“ask, show, tell, advise, inform, teach”等，可以先有个宾语，然后才接着加上适当的“疑问词+不定式动词” 结构。例如：
● The chief technician showed the apprentice how to repair the machine.
● Have you told him where to get the application form?
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