To see is to believe. (1). 主语 It’s important to learn . 作主语时,往往放在谓语之后, 用it作形式主语. (2). 表语 My job is to teach students. Your task is to clean the classroom. (3) 宾语 I want to go home. I found it necessary to talk to him again. 常用动词不定式作宾语的动词有： hope, refuse, learn, set out ,choose, decide, agree, manage, pretend, plan.
+ adj.+ to do
warn, tell, allow, help, ask, would like, (4). 宾补 wish, order, send, lead,force等 The teacher told me to clean the blackboard. I expect you to give me some help.
注意：当不定式放在下列动词后面作宾语补语 时，不定式不带to。 listen to
二听： 五看: let 三使： make have hear watch see look at observe notice
I made him do his work. He was made to do his work (by me).
(5). 定语 I have something to say. He has a lot of homework to do . He is looking for a room to live in . There is nothing to worry about. (如果不定式中的动词是不及物动词, 则不定式中要有介词.) something, anything, nothing, everything等 复合不定代词常用不定式做后置定语。 请给我一支写字的笔. Please give me a pen to write with.
表示目的、原因、结果或条件。 I came here to see you. （目的） He got up early to catch the train. We are sorry to hear that. (原因) He hurried there to find nobody there . (结果) To look at him，you will be angry． (条件)
?不定式与疑问词who,which, when, how, what 等连 用，在句中起名词作用，可充当主语、表语、宾语等。
He didn’t know what to say. (宾语) How to do it is very important. (主语) My question is when to start. (表语)