（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、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.
（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，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。
（注）：
①上述感觉动词与使役动词转换为被动结构时。其后的不定式一般需带to，如：
He was seen to come.
The boy was made to go to bed early.
②在动词find与help之后，不定式可带to亦可不带to，如：
He was surprised to find the sheep （to） break fence at this season.他发现羊在此季节越出栅栏，感到惊讶。
3）在do nothing／anything／everything but（except）结构中。例如：
Last night I did nothing but watch TV.昨天晚上，我除了看电视别的什么也没有干。
但是，如果谓语动词不是“do nothing，anything，everything”，那么but（except）所跟的不定式则仍须带。
The doctor told him nothing but to stop smoking.医生除了让他戒烟，其它什么都没有说。
There was nothing for them to do but to remain silent.除了保持沉默以外，他们没有别有别的办法。
（4）不定式与动名词的逻辑主语和分词独立结构
1）不定式的逻辑主语为：for +名词（或代词宾格）+ 不定式。例如：
I found it impossible for him to do the job alone.我发现他—个人干这活是不可能的。
（注）在表示人物性格、特性等的形容词后面，常用of引出不定式的逻辑主语。例如：
It was wise of him to do that.他那样做是明智的。
2）动名词的逻辑主语为；①人称代词的所有格+动名词；②名词‘s+动名词。例如：
Tom insisted on my going with them.他坚持要我和他们一起去。
He dislikes his wife's working late.他不喜欢他妻子工作得很晚。
3）某些形容词，如：careless等不定式后可以加of来引导出其逻辑主语。这类词主要有：absurd, bold, brave, courageous, careful, careless, clever, wise, foolish, silly, stupid, good, nice, kind thoughtful, considerate, greedy, generous, honest, modest, polite, rude, cruel ,selfish, wicked, wrong等
It is very kind of you to help me.你帮助我太好了。
间或也可用for + there to be表示（而且there后面的不定式只能是to be）。
It's a great pity for there to be much trouble in the company.太遗憾了公司里有这么多的麻烦。
