不定式作主語
To +動詞原形
Not to do
解釋：具有動詞的性質(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：to be doing
完成時：to have done
完成進(jìn)行時：to have been doing
to do:表示動作發(fā)生在謂語之後，I want to go home.與謂語動詞幾乎同時發(fā)生
to be doing :表示與謂語動詞同時發(fā)生
to have done:發(fā)生在謂語動詞之前 Someone was considered to have done something.
to have been doing :動作從過去開始一直持續(xù)到謂語動詞這個動作的發(fā)生
（1）動詞不定式作主語時，謂語動詞常常用單數(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）如果動詞不定式太長，常常用it作形式主語，而將真正的主語——動詞不定式後置。例如：
It took me half an hour to walk there．我走到那兒花了半小時的時間。
It's important for us to learn English well．對我們來說，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 表示贊敭或批評的詞。在不定式前的sb，可看作其邏輯主語。這一句式有時相儅於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.