动词不定式
1、不定式做主语一般表示具体的某次动作。
To complete the 30-storied building in one year was quite a difficult task.
To do that implies taking responsibility.
当主语较长，谓语较短时，常用it做形式主语，而将不定式放到谓语的后面。如：
It is important for modern young people to master at least two foreign languages.
It做形式主语，不定式放在谓语动词之后常用于下列结构中：
A. It is+形容词+(for/of sb.)+不定式，如：
It is not easy to catch fish with your hands only.
It is important for us young people to learn English and master it.
B. It is+名词+不定式
It is a pity to have to go without her.
C. It takes (sb.) some time +不定式
It takes me three hours to learn English each day.
2、不定式作表语
不定式作表语常表示将来的动作，主语常常是表示意向、打算、计划的词(idea, task, purpose)等,如：
The most important thing for one’s health is to have plenty of exercise.
3、不定式作宾语
I like to go out for walks in the warm sunshine in spring.
She failed to finish the assignment in time, and she was worried about it.
4、“wh-+不定式”结构
不定式前加一个疑问代词(what, which, who, whom, whose)或疑问副词(where, when, how, why)，以及连词whether构成特殊的不定式短语，其作用相当于一个从句，这样的不定式短语常常在某些动词后面作宾语。
I couldn’t decide which book to choose.
I can tell you where to get this book.
“wh-word+不定式”结构除了作宾语外，还可以在句中作主语或者表语。如：
When to start the program remains undecided.
The question is how to put the plan into practice.