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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?

5.作定语。

a.与被修饰词有动宾关系。如：I have something important to tell you.但如果不定式动词为不及物动词，后面的介词千万不要省略。

如： Maybe they have three rooms to live in.

b.与被修饰词有主谓关系。

如： Mr Liang is always the first to come and the last to leave.

c.与被修饰之间只有修饰关系。

如： I have no time to play cards.

6.作状语，表示目的、原因、方法、方向、结果等。

如： I'll go to meet my friend at the railway station.

7.不定式复合结构"for sb. to do sth" 作主语时，常用"It is +adj+ for

of sb. to do sth"的句式。形容词good, bad, polite, unkind, kind, ice, clever,

right, wrong, careful等用"It is +adj +of sb. to do sth."

其他形容词用 for。

如：

It's dangerous for you to ride so fast.

It's very kind of you to help me.

8.动词不定式与疑问句who, what, which, when, how, where, whether等连用。

如： I don't know when to start.

He didn't tell me where to go.

但上面结构相当于一个从句，故上述句子也可表达为：

I don't know when we'll start.

He didn't tell me where he would go.

注意：

a.有些动词或动词短语不能带不定式，只能接动词的-ing形式。

如： enjoy, finish, keep, mind, miss(错过),be busy, go on, keep on, be worth, practise等。

如： The peasants are busy picking apples.

Would you mind my opening the door?

b.有些动词后可接不定式，也可接动词的-ing形式，但意思不同。

如： Lu Jian forgot to post the letter.(该寄但还没做)

Lu Jian forgot posting the letter.(已经寄过信了)

They stopped to sing a song.(停止在做的工作而去做另一件工作)

They stopped singing.(停止正在做的工作)

以上就是九年级英语动词知识点动词不定式的形式讲解的全部内容，希望可以帮助到大家的学习!