英語有個很有用的結構，就是：疑問詞+不定式動詞(question word + to-infinitive)。

疑問詞是疑問代詞“who, whom, what, which, whose”和疑問副詞“when, where, how, why ”。此外，連接詞“whether”也適用。

“疑問詞+不定式動詞”結構有何功用呢?其主要功用有下列五種：

⑴當主語，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當賓語，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當補足語，如：

● The problem is where to find the financial aid.

● The question is who to elect.

⑷當名詞同位語，如：

● Tom had no idea which book to read first.

● Do you have a rough impression how to do it?

⑸當賓語補足語，如：

● Jim is not sure whose to choose.

● Mary and John are not certain whether to get married or not.

適用于“疑問詞+不定式動詞”的動詞包括：“know, see, decide, tell, ask, consider, discover, explain, forget, guess, hear, imagine, inquire, learn, remember, think, wonder, understand”等。

有點值得特別注意的是：當這結構當賓語時，它的作用等于名詞分句，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.

有些動詞，如“ask, show, tell, advise, inform, teach”等，可以先有個賓語，然后才接著加上適當的“疑問詞+不定式動詞” 結構。例如：

● The chief technician showed the apprentice how to repair the machine.

● Have you told him where to get the application form?

綜上所述，可見“疑問詞+不定式動詞”結構是個形象清新，既容易用又不容易出錯的特別結構，可以多多運用。