1、做主语

相当于名词的功能。如：To live of money is the root of evil.这个句子中to live of money是不定式短语做主语。

例句：

To see is to believe．

 眼见为实。

 To act like that is foolish．

 这样做是愚蠢的

2、做表语：

a teacher.不定式to become a teacher放在was 后面。

例句：

My chief purpose is to point out the difficulties of the matter.

我的主要目的是指出这件事的困难。

What I would suggest is to put off the meeting.

我建议推迟会议。

 3、做宾语

例如：I try to keep that in mind.在这里to keep that in mind做try的宾语。

例句：

 She longed to go back to her hometown.她渴望返回家乡。

 I can't bear to look back.往事不堪回首。

He swore to abide by this principle.他发誓恪守这一原则

4 、做宾语的补足语

补充说明宾语的作用。如：I know nothing about him and I beg you to tell me nothing. 这个句子的to tell me nothing用来补充说明you 的做法。

例句：

I here them sing yesterday. 昨天我听见他们唱歌了。           

Did you see him go out? 你看见他们出去了吗。          

I felt something crawl up my leg. 我感到有什么东西爬道我腿上了。

5、做主语的补足语

He was seen to enter the laboratory.有人看见他进了实验室。不定式to enter the laboratory用来补充说明主语He进去了实验室。