不定式分句可以在句子中作主语和主语补语、宾语和宾语补语、状语等。具体而言：

1. 作主语和主语补语：

To finish the work is very easy.

完成这项工作很容易。(=It is very easy to finish the work.)

Last year, the wisest policy was not to interfere any more.去年最明智的政策就是不再干预。

2. 作宾语和宾语补语：

I learned to speak English when I was young. 我小时候就学习说英语了。

The teacher’s greatest wish is to see his student pass the final exam.这个老师最大的心愿就是看到学生们通过期末考试。

My teacher advised me to study abroad for further education.老师建议我留学深造。

3. 作状语：

(1)表目的：

She opened the window to get more fresh air. 她打开窗户呼吸更多新鲜空气。

(2)表结果：

He hurried to the station only to be told that the train had left.

他匆忙赶到车站，结果火车已经走了。

(3)表原因：

I was really lucky to pass the final exam. 能通过考试，我真的很幸运。

I am sorry to hear that news. 听到这则消息，我感到很抱歉。

由上可知，不定式分句作为一种分句结构，主要有以上三种句法功能。当然，它还可以在名词词组中作修饰语，在形容词词组中作补足成分，这里就不一一赘述。