1．作主语。 如：


　　To learn English is very important.


　　但实际上不定式作主语常用 it来作形式主语，而将不定式移至谓语动词后作真正的主语。


　　如上句可表达为：


　　It's very important to learn English.


　　2．作表语。 如：


　　My idea is to ring him up at once.
 

　　3．作宾语。 如：


　　I have decided to go to Binjiang Primary School.


　　4．作宾语补足语。


　　a. ask, want, teach, tell, know, would like, allow等动词后面接动词不定式作宾语补足语。


　　如： The policemen asked him to get off the bus.


　　b. hear, see, look at, feel, watch, notice, listen to等动词后接不带to的动词不定式作宾语补足语。


　　如： We often see Miss Li clean the classroom.


　　c. let, make, have这些使役动词后接不带to的动词不定式作宾语补足语。但在被动语态中这些不带to的都须带上to。


　　如： In those days the bosses often made the workers work day and night.


　　d.动词help接动词不定式作宾语补足语，可带to也可不带to。


　　如： Can you help me (to) carry the heavy bag?