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.

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。

解析：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。