非谓语动词
在句中充当除谓语以外的句子成分的动词形式叫做非谓语动词。非谓语动词分为三种形式：不定式、动名词和分词（分词包括现在分词和过去分词）
不定式
一、不定式的作用
1、作主语不定式作主语时，谓语用单数。往往用it 作形式主语，把不定式放在谓语后面。如：
It took us two hours to finish the job.
It is impossible for us to get there on time.
It is very kind of you to help us.
注意：（1）其他系动词如look, appear等也可用于此句型。
（2）当不定式作主语的句子中又有一个不定式作表语时，不能用It is …to…的句型。试比较：It is to negate my own idea to believe him.（错）
To believe him is to negate my own idea .（对）
（3）It is+ adj. of / for sb. to do sth. 结构中，当不定式的逻辑主语和前面的形容词可以构成系表结构时，用of, 否则用for.
2、作宾语
（1）动词+不定式。如：
He managed to escape from the fire.
I find it hard to get along with him.（it 作形式宾语）
注：下列动词通常用不定式作宾语：want, try, hope, wish, need, forget, know, promise, refuse, help, decide, begin,
start, learn, agree, choose, get等
（2）动词+疑问词+to , “特殊疑问句+不定式”相当于名词，作宾语。如：
I don’t know what to do next/ how to do it next.
I can’t decide when to go there.
注意：不定式短语作宾语时，如果还带有宾语补足语，往往把不定式短语放在宾语补足语之后，用it 作形
式宾语。如：I find it necessary to learn a foreign language.
3、作宾语补足语
（1）动词+宾语+不定式（to do ）。如：
He warned me to be careful.
I want you to speak to Tom.
What makes you think so?（不带to的不定式）
注：可以用动词不定式做宾补的动词有：ask, tell, order ,want ,get, would like, like, advise, invite, allow, help, wish,warn, expect, prefer, encourage
（2）表见解、看法的动词结构可为：动词+宾语+ to be 的不定式结构。如：
We consider Tom to be one of the best students in our class.
The book is believed to be useful.（被动语态）
(3) There +不定式。如：
We didn’t expect there to be so many people there.我们没料到会有那么多人在那里。
注意：（1）有些动词需用as 短语作补语，像regard, think, believe, take, consider。如：
We regard Tom as our best teacher. 我们认为汤姆是我们最好的老师。
Mary took him as her father. 玛丽把他当作自己的父亲。
（2）在动词feel （一感），hear, listen to（二听），have, let, make（三让），notice, see, watch, observe, look at（五看）(即：吾看三室两厅一感觉)等后面的补足语中，不定式不带to，但变为被动语态后，必须带to。如：
They saw the boy fall off the tree. The boy was seen to fall off the tree.
(3)help后面作宾语补足语的不定式可以带to,也可以不带to.
I often help him （to）clean the room.
I helped him (to) find his things.
4、作定语
不定式作定语，修饰名词或代词，不定式和它所修饰的名词之间有逻辑上的主谓关系、动状关系、同位关系或动宾关系。不定式通常要放在这些被修饰的词后。如：
I have a lot of work to do.（动宾关系）
He is looking for a room to live in.（动状关系）
He is the first person to think of the idea.（主谓关系）
He has got a chance to go abroad.（同位关系）
注意：1.不定式的逻辑主语是句子的主语时，不定式用主动形式表被动。如：
Do you have anything else to say?
2.如果作定语的不定式是一个短语，则要保留不定时短语中的副词或介词。如：
I need a pen to write with . (I will wirte with the pen ) (我需要一直钢笔写字)
I have a little baby to look after .(I must look after the little baby ) (我有一个婴儿要照看)
作状语，表示目的、结果、原因等，有时还有一些固定搭配的不定式短语，如in order to , so as to, so…as to, such …as to, ….enough to, too…to等。
