不定式作主語
To +動(dòng)詞原形
Not to do
解釋：具有動(dòng)詞的性質(zhì)，作賓語，狀語/具有名詞的性質(zhì)，作主語/具有形容詞性質(zhì)，作定語/具有副詞的性質(zhì)，作狀語。除謂語不可作之外，其他都可。
一般式：to do
進(jìn)行時(shí)：to be doing
完成時(shí)：to have done
完成進(jìn)行時(shí)：to have been doing
to do:表示動(dòng)作發(fā)生在謂語之後，I want to go home.與謂語動(dòng)詞幾乎同時(shí)發(fā)生
to be doing :表示與謂語動(dòng)詞同時(shí)發(fā)生
to have done:發(fā)生在謂語動(dòng)詞之前 Someone was considered to have done something.
to have been doing :動(dòng)作從過去開始一直持續(xù)到謂語動(dòng)詞這個(gè)動(dòng)作的發(fā)生
（1）動(dòng)詞不定式作主語時(shí)，謂語動(dòng)詞常常用單數(shù)。例如：
To do morning exercises is useful for our health. 做早操有利於我們的健康。
To sweep the floor is my duty every day. 每天打掃地板是我的責(zé)任。
（2）如果動(dòng)詞不定式太長，常常用it作形式主語，而將真正的主語——動(dòng)詞不定式後置。例如：
It took me half an hour to walk there．我走到那兒花了半小時(shí)的時(shí)間。
It's important for us to learn English well．對(duì)我們來說，學(xué)好英語是重要的。
下麪歸類五種句型：
①It+be+名詞+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+形容詞+for sb+to do
It is difficult for us to finish writing the composition in a quarter of an hour.
④It+be+形容詞+of sb+to do
It is stupid of you to write down everything (that) the teacher says.
⑤It seems(appears)+形容詞+to do
It seemed impossible to save money.
注意：
在句型③中，常用表示客觀情況的形容詞，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 表示贊敭或批評(píng)的詞。在不定式前的sb，可看作其邏輯主語。這一句式有時(shí)相儅於Sb is+形容詞+to do句式。
如：
It's kind of you to help me with my English.=You are kind to helpme with my English.