1）目的状语
常用结构为to do , only to do（仅仅为了）, in order to do, so as to do, so（such）… as to…（如此…以便…）。例如：
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，可以表示没有预料到的或事与愿违的结果，不定式要放在句子后面。
I awoke to find my truck gone. 我醒来发现箱子不见了。
He searched the room only to find nothing. 他搜索了房间，没发现什么。
3） 表原因
I'm glad to see you. 见到你很高兴。
She wept to see the sight. 她一看到这情形就哭了。
4）表示理由和条件
He must be a fool to say so.
You will do well to speak more carefully.