动词不定式的作主语
动词不定式作主语时，句子的谓语动词常用单数，其位置有以下两种：
(1)把不定式置于句首。如：
To get there by bike will take us half an hour.骑自行车到那儿将花费我们半个小时。
To make up for lost time is not possible.要弥补失去的时光是不可能的。
To lean out of the window is dangerous.身子探出窗外很危险。
To save money now seems impossible.现在好像不可能存钱。
(2)用it作形式主语，把真正的主语不定式置于句后，常用于下列句式中。如：
①It+be+名词+to do
It's our duty to take good care of the old.照顾老人是我们的责任。
It is an offence to drop litter in the street.在马路上乱丢废物是违章的。
②It takes sb+some time+to do
How long did it take you to finish the work?你花了多少时间来完成这项工作？
③It+be+形容词+for sb+to do
It is difficult for us to finish writing the composition in a quarter of an hour.我们难以在四分之一小时内完成这篇作文。
It is stupid of you to write down everything (that) the teacher says.你把老师说的所有东西都记下来的行为是很愚蠢的。
④It seems(appears)+形容词+to do
It seemed impossible to save money.
在句型③中，常用表示客观情况的形容词，如：difficult,easy,hard,important,impossible,necessary 等；在句型④中，常用careless,clever,good,foolish,honest,kind,lazy,nice,right,silly,stupid,wise等 表示赞扬或批评的词。在不定式前的sb，可看作其逻辑主语。这一句式有时相当于Sb is+形容词+to do句式 ，如：It's kind of you to help me with my English.=You are kind to help me with my English.
⑤It + 不定式结构可位于believe/consider/discover/expect/find/think和wonder之后
He thought it would be safer to go by train.他认为乘火车比较安全。
He will find it is hard to make friends.他会感觉到交朋友困难。
⑥不定式的完成式也可作句子的主语
To have made the same mistake twice was unforgivable.两次犯同样的错误是不可原谅的。
It is better to have loved and lost than never to have loved at all.爱过而后失去了爱，比从未爱过要好。
(3)举例
（1） It's easy (for me) to do that.我做这事太容易了
easy, difficult, hard, important, possible, impossible, comfortable, necessary, better;
the first, the next, the last, the best, too much, too little, not enough
It's so nice to hear your voice.听到你的声音真高兴。
It's necessary for you to lock the car when you do not use it.当你不用车的时候，锁车是有必要的。
（2） It's very kind of him to help us. 他帮助我们，他真好。
Kind, nice, stupid, rude, clever, foolish, thoughtful, thoughtless, brave, considerate(考虑周到的), silly, selfish(自私的)
例句:
It was silly of us to believe him. 我们真愚蠢，竟然相信了他。
It seemed selfish of him not to give them anything. 他不给他们任何东西，这显得太自私了。
注意：（1） 其他系动词如，look，appear等也可用于此句型
（2） 不定式作为句子成分时，动词用单数形式。
（3） 当不定式作主语的句子中又有一个不定式作表语时，不能用It is… to…的句型
(对)To see is to believe. 眼见为实。
(错)It is to believe to see.
