非谓语动词常考的其它结构
(1)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、的宾语、的表语和双重宾语。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.
(2)介词except和but作“只有…，只能…”讲时跟不定式结构(but与不带to的不定式连用)。
When the streets are full of melting snow, you cannot help but getting your shoes wet.
(3)不带to的不定式
1)在表示生理感觉的动词后的不定式不带to。这类词有：
feel 觉得 observe 注意到医学全在,线www.med126.com，看到 overhear听到
watch注视 listen to听 perceive察觉，感知
notice注意 see看见 look at看 hear听
On seeing the young child fell into the lake，Eric sprang to his feet，and went on the rescue.
2)另一类是某些使役动词，如make, let，have等。如：
Let him do it.让他做吧。
I would have you know that I am ill.我想要你知道我病了。
