疑问词是疑问代词“who, whom, what, which, whose”和疑问副词“when, where, how, why ”。此外，连接词“whether”也适用。
● 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.
● we must know what to say at a meeting.
● he could not tell whom to trust.
● do you know how to play bridge?
● the problem is where to find the financial aid.
● the question is who to elect.
● tom had no idea which book to read first.
● do you have a rough impression how to do it?
● 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。
● 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?