不定式或不定式短语的用法：

不定式在句子中可作主语、表语、宾语、定语、状语和补足语。

1．作主语

不定式或不定式短语作主语时，谓语动词用第三人称单数。如：

To serve the people well is our duty.

To get there on foot will take several hours.

动词不定式短语作主语时，往往放在谓语动词的后面，用引导词it作形式主语。上述两句也可改为：

It is our duty to serve the people well.

It will take several hours to get there on foot.

(take – need需要(多少时间等))

类似这样不定式做主语的句型:

It is + n.+ to do

It a\takes sb.+ some time + to do

It is +adj.+ for sb.+ to do

It is +adj.+ of sb.+ to do

It is our duty to help them.

It will take you quite a long time to finish it.

(finish - fulfill完成)

It is easy for us to answer this question.

It is kind of you to help me

(help –aid 帮助)