不定式的复合结构：for...to do sth
不定式的逻辑主语明确是谁时,是不用复合结构的,即：to do sth ；否则,用复合结构.
不定式的句法作用举例如下：
1.主语:It is very important for you to study English.
2.表语:The car is for you to drive.
3.状语:The man works hard for his boss to be pleased.
4.定语：There is no need for you to go to the office.
5.宾语：I've arranged for her to see you off.