不定式作表语

不定式可放在be动词后面，形成表语。例如：

My work is to clean the room every day.

His dream is to be a doctor.

不定式作定语

不定式做定语通常要放在被修饰的词后。例如：

I have a lot of work to do.

So he made some candles to give light.

不定式作状语

1)目的状语

To… only to (仅仅为了),in order to,so as to,so(such)… as to… (如此……以便……)

He ran so fast as to catch the first bus.他飞快地跑以便赶上第一班车。

I come here only to say good-bye to you.我来仅仅是向你告别。

2)作结果状语，表事先没有预料到的，要放在句子后面。

What have I said to make you angry.

He searched the room only to find nothing.

3)表原因

I’m glad to see you.

典型例题

The chair looks rather hard, but in fact it is very comfortable to ___.

A. sit B. sit on C. be seat D. be sat on

答案：B.如果不定式为不及物动词，其后应有必要的介词。当动词与介词连用时，常位于"形容词+动词不定式"结构的末尾。