1．作主語(yǔ)。 如：
To learn English is very important.
但實(shí)際上不定式作主語(yǔ)常用 it來(lái)作形式主語(yǔ)，而將不定式移至謂語(yǔ)動(dòng)詞后作真正的主語(yǔ)。
如上句可表達(dá)為：
It's very important to learn English.
2．作表語(yǔ)。 如：
My idea is to ring him up at once.
3．作賓語(yǔ)。 如：
I have decided to go to Binjiang Primary School.
4．作賓語(yǔ)補(bǔ)足語(yǔ)。
a. ask, want, teach, tell, know, would like, allow等動(dòng)詞后面接動(dòng)詞不定式作賓語(yǔ)補(bǔ)足語(yǔ)。
如： The policemen asked him to get off the bus.
b. hear, see, look at, feel, watch, notice, listen to等動(dòng)詞后接不帶to的動(dòng)詞不定式作賓語(yǔ)補(bǔ)足語(yǔ)。
如： We often see Miss Li clean the classroom.
c. let, make, have這些使役動(dòng)詞后接不帶to的動(dòng)詞不定式作賓語(yǔ)補(bǔ)足語(yǔ)。但在被動(dòng)語(yǔ)態(tài)中這些不帶to的都須帶上to。
如： In those days the bosses often made the workers work day and night.
d.動(dòng)詞help接動(dòng)詞不定式作賓語(yǔ)補(bǔ)足語(yǔ)，可帶to也可不帶to。
如： Can you help me (to) carry the heavy bag?
5．作定語(yǔ)。
a.與被修飾詞有動(dòng)賓關(guān)系。如：I have something important to tell you.但如果不定式動(dòng)詞為不及物動(dòng)詞，后面的介詞千萬(wàn)不要省略。
如： Maybe they have three rooms to live in.
b.與被修飾詞有主謂關(guān)系。
如： Mr Liang is always the first to come and the last to leave.
c.與被修飾之間只有修飾關(guān)系。
如： I have no time to play cards.
6．作狀語(yǔ)，表示目的、原因、方法、方向、結(jié)果等。
如： I'll go to meet my friend at the railway station.
7．不定式復(fù)合結(jié)構(gòu)"for sb. to do sth" 作主語(yǔ)時(shí)，常用"It is +adj+ for
of sb. to do sth"的句式。形容詞good, bad, polite, unkind, kind, ice, clever,
right, wrong, careful等用"It is +adj +of sb. to do sth."
其他形容詞用 for。
如：
It's dangerous for you to ride so fast.
It's very kind of you to help me.
8．動(dòng)詞不定式與疑問(wèn)句who, what, which, when, how, where, whether等連用。
如： I don't know when to start.
He didn't tell me where to go.
但上面結(jié)構(gòu)相當(dāng)于一個(gè)從句，故上述句子也可表達(dá)為：
I don't know when we'll start.
He didn't tell me where he would go.
注意：
a.有些動(dòng)詞或動(dòng)詞短語(yǔ)不能帶不定式，只能接動(dòng)詞的-ing形式。
如： enjoy, finish, keep, mind, miss(錯(cuò)過(guò)),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.有些動(dòng)詞后可接不定式，也可接動(dòng)詞的-ing形式，但意思不同。
如： Lu Jian forgot to post the letter.(該寄但還沒(méi)做)
Lu Jian forgot posting the letter.(已經(jīng)寄過(guò)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)
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