不定式的功能 

1.作主语——不定式作主语时一般看作单数 

例句：To be a teacher is his dream. 

不定式作主语时可以用it作形式主语，真实主语放在后面。 

例句：It is nice to meet you. 

2.作表语 

例句：His dream is to open a shop of his own. 

His theory proved to be true. 

3.作宾语 

例句：He refused to give me a hand. 

Jim failed to pass his English exam. 

4.作宾语补足语 

例句：The teacher advised me to read more. 

一些感官动词或使役动词后面的不定式宾语补足语要省略to，比如see, look at, notice, watch, observe, hear, listen to, feel 

例句：Don't let the children play in the street. 

5.作定语——放在被修饰词之后 

例句：He worked out a plan to help his brother. 

Jim was the first to come this morning. 

6.做状语 

1）目的状语 

例句：He ran so fast as to catch the first bus. 

2）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。 

例句：He searched the room only to find nothing. 

3）原因状语 

例句：She cried to see the sight. 

4）条件状语 

例句：He must be a fool to say so. 

You will do well to speak more carefully.