不定式作表語

不定式可放在be動詞后面，形成表語。例如：

My work is to clean the room every day.

His dream is to be a doctor.

不定式作定語

不定式做定語通常要放在被修飾的詞后。例如：

I have a lot of work to do.

So he made some candles to give light.

不定式作狀語

1)目的狀語

To… only to (僅僅為了),in order to,so as to,so(such)… as to… (如此……以便……)

He ran so fast as to catch the first bus.他飛快地跑以便趕上第一班車。

I come here only to say good-bye to you.我來僅僅是向你告別。

2)作結果狀語，表事先沒有預料到的，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.

典型例題

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.如果不定式為不及物動詞，其后應有必要的介詞。當動詞與介詞連用時，常位于"形容詞+動詞不定式"結構的末尾。