中高考：探究动词不定式作为主语的重要性
一、什么是动词不定式呢？
1.动词不定式的基本形式是“to+动词原形”，有时可以不带to。
2.不定式在句中不能单独作谓语，是一种非谓语动词结构。
3.不定式保留动词的某些特性，可以有自己的宾语、状语等。
4.动词不定式可以作句子的主语、表语、宾语、定语、补语、状语。
动词不定式例子：
to live in Fuzhou
to read a book
to talk with the boy
to learn English well
二、不定式作主语
To learn English wellis very important. 学好英语很重要。
To try our best to deal with these problems is our duty.
To climb the mountainis very difficult for us.对我们来说爬这座山很困难
To get to the bus stationtakes me two hours.
花费我两个小时到达公交车站。
To seeis to believe. 眼见为实
How to learn English wellis important. 如何学好英语很重要。
1. 动词不定式作主语，谓语动词用第三人称单数形式。
2.直接把动词不定式置于句首的情况不多。多数情况下，如果作主语的动词不定式太长，为了保持句子的平衡(避免句子头重脚轻)，常用it作形式主语，代替真正的主语──动词不定式。而真正的主语──动词不定式，则放在句子的末尾。
It is very important to learn English well
形式主语真正的主语
It is our duty to try our best to deal with these problems
形式主语 真正的主语
It is very difficult for usto climb the mountain
形式主语 真正的主语
It is very kind of youto help me with English
形式主语真正的主语
It takes me two hoursto get to the bus station.
形式主语真正的主语
备注：辨析句型
Its + adj. + of sb.+ to do sth. 与 Its + adj. + for sb.+ to do sth.
一般来说，of sb.前面的形容词是careful, careless,clever, foolish, polite,impolite,lazy,good, kind, nice, wise等，主要说明动词不定式的行为人的性格、品质；for sb.前面的形容词是dangerous, difficult, easy, hard, heavy, important, interesting, necessary等，一般说明不定式动作的特性。
Its very kind of you to help me. 你能帮助我，真是太好了。
Its foolish of him to make such mistakes.他真傻啊，竟然犯这种错误。
Its dangerous for you to swim alone.你自己一个人去游泳很危险。
Its very difficult for us to climb the mountain.对我们来说爬这座山很困难。
还有一个区别是：在of sb. 中，可用sb.直接作主语来改写句子。而for sb.结构是不可以这样转换的。
Its very kind of you to help me. = You are very kind to help me.
练习：
1. Is ______ necessary to return the book tomorrow ?
A. this B. that C. it D. which
2. It’s necessary ____ us ____ wear suitable clothes on every occasion.
A. for; to B. for; for C. of ; to D. to; for
3.It is very kind_____you to help me with my English.
Its hard____ me to learn it well.
A. for, of B. of, ofC. of, forD. for , for
4.Its important______the piano well.
A. for him to play B. of him playing
C. of him to play D. for him playing
5.It is difficult______us to finish this work in an hour.
A. forB. of C. withD. to
6.Its clever_____you to work out the math problem.
A. forB. of C. withD. to
参考答案：
1.C ， 2.A，3.C，4.A，5.A,6.B