“疑问词+不定式(短语)”是一个很有用的结构，它的作用相当于一个名词性从句，通常在某些动词(如tell, know, imagine, learn, decided, forget, remember, consider, understand等)后面做宾语，有时也用作主语、表语、同位语等。如：　　

He didn’t know how to answer her. 他不知道该怎样回答她。　　

When to start has not been decided. 何时动身尚未决定。　　

I don’t know who to ask advice from. 我不知道应向谁征求意见。　　

Would you please advise me which to buy? 你能否告诉我该买哪一个才好?　　

Please explain (to me) where to begin and how to do it. 请 (给我)解释该从哪里开始及该怎么做。　　

They exchanged views on the question of whom to elect. 他们就该选谁的问题交换了意见。　　

注意：按英语习惯，疑问词why后面通常不接不定式。但在以why或why not 开头的省略型疑问句中，其后可跟不带to 的不定式。如：　　

Why get upset just because you got a bad mark? 何必因为没有考好就想不开呢?　　

You’re looking tired. Why not take a holiday? 你一脸倦容，怎么不休假呢?