考博英语语法学习备考：疑问词+不定式结构
疑问词who，what，which，when，where和how后加不定式可构成一种特殊的不定式短语。它在句中可以用作主语、宾语、表语和双重宾语。如：
When to start has not been decided.何时动身尚未决定。(主语).
I don't know what to do.我不知道该怎么办。(宾语).
The difficulty was how to cross the river.困难在于如何过河。(表语).
I can tell you where to get this book.我可以告诉你哪里可以买到此书。(双重宾语).
(注)A.有时疑问词前可用介词，如：
I have no idea of how to do it.我不知道如何做此事。
B.动词know 后面不能直接跟不定式作宾语，只能跟疑问词(如：how, what)+不定式：
While still a young boy, Tom knew to play the piano well and as he grew older, he wrote operas, the most famous of which is Carmen.