不定式作主语

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

（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。

（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 helpme with my English.