To 动词原形
Not to do
解释：具有动词的性质，作宾语，状语/具有名词的性质，作主语/具有形容词性质，作定语/具有副词的性质，作状语。除谓语不可作之外，其他都可。
一般式：to do
进行时：to be doing
完成时：to have done
完成进行时：to have been doing
to do:表示动作发生在谓语之后，I want to go home.与谓语动词几乎同时发生
to be doing :表示与谓语动词同时发生
to have done:发生在谓语动词之前 Someone was considered to have done something.
to have been doing :动作从过去开始一直持续到谓语动词这个动作的发生
To finish the task will take a long time. 要完成这项任务将要花费很长时间。
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。
（1）动词不定式作主语时，谓语动词常常用单数。例如：
To do morning exercises is useful for our health. 做早操有利于我们的健康。
To sweep the floor is my duty every day. 每天打扫地板是我的责任。
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）如果动词不定式太长，常常用it作形式主语，而将真正的主语——动词不定式后置。例如：
It took me half an hour to walk there．我走到那儿花了半小时的时间。
It's important for us to learn English well．对我们来说，学好英语是重要的。
下面归类五种句型：
①It be 名词 to do
It's our duty to take good care of the old.
②It takes sb some time to do
How long did it take you to finish the work?
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.
④It be 形容词 of sb to do
It is stupid of you to write down everything (that) the teacher says.
⑤It seems(appears) 形容词 to do
It seemed impossible to save money.
注意：
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.
小结：
常用不定式做主语的句型有：
（1）It’s difficult (important, necessary) for sb. to do
这里的形容词常用：easy, difficult, hard, important, possible, impossible, comfortable, necessary, better;
the first, the next, the last, the best, too much, too little, not enough
（2）It’s kind (good, friendly, polite, careless, rude, cruel, clever, foolish, brave) of sb. to do.
这里的形容词常用：kind, nice, stupid, rude, clever, foolish, thoughtful, thoughtless, brave, considerate(考虑周到的), silly, selfish(自私的)
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。
常用动名词做主语的句型有：
It’s no good (use, fun) doing.
It’s (a) waste of time one’s doing.
知识拓展：不定式和动名词作主语的区别
(1) 动名词作主语通常表示抽象动作；而不定式作主语表示具体动作。
Smoking is prohibited here．这里禁止抽烟。(抽象)
It is not very good for you to smoke so much．你抽这么多烟对你身体很不好。(具体)
(2) 动名词作主语时，通常用以表示一件已知的事或经验。不定式短语通常用来表示一件未完成的事或目的。
Climbing mountains is interesting．爬山很有趣。(经验)
Driving a car during the rush hour is tiring．在高峰时刻开车令人厌烦。(经验)
To finish the task will take a long time. 要完成这项任务将要花费很长时间。（具体）
(3) 不定式做主语，一般用it当形式主语，把作主语的不定式短语后置。
It took me only five minutes to finish the job.
（4）动名词做主语往往表示普通的，一般的行为；不定式作主语常常表示某次具体的行为。
Collecting information about children's health is his job. 收集有关儿童健康的It’s necessary to discuss the problem with an experienced teacher. 与一位有经验的老师讨论这个问题是有必要的。信息是他的工作。
