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、动名词和现在分词都有时态和语态的变化.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 manage d 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 warn ed 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r egard, 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。
Mar y 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 t o 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。
（1）做目的状语，to, only to （仅仅为了）, in order to, so as to, so(such)….as to…（如此···以便···）如：He ran so fast as to catch the first bus. 他飞快地跑以便赶上第一班车。
He came to the school to see his son.（2）作结果状语，表事先没有预料到的，要放在句子后面。
如：He hurried to the post office, only to find it was closed.He searched the room ,only to find nothing.（3）做原因状语。
如：We were very excited to hear the news.I’m glad to see you.（4）做条件状语。
如：To turn to the left , you could find a post office.5、作表语不定式可放在be动词后面，构成表语。
如：The question is how to put it into practice.My question is when to leave.His dream is to be a doctor.Her work is to look after the babies.注意：1.不定式在句中作表语时，对应的谓语动词用单数。
2.当助于是不定式时，表语不能用Ving形式，可用不定式。
如：To see is to believe. （眼见为实）6、独立结构。
如：To tell you the truth, I don’t agree with you.To make matters worse, it began to rain.二、不定式的时态和语态1、不定式的时态（1）现在时：有时与谓语动词表示的动作同时发生，有时发生在谓语动词的动作之后。
如：He seems to know this. I hope to see you again.（2）完成时：表示的动作发生在谓语动词表示的动作之前。
如：I’m sorry to have given you so much trouble.He seems to have caught a cold.（3）进行时：表示动作正在进行，与谓语动词表示的动作同时发生。
如：He seems to be eating something.（4）完成进行时：表示动作从过去某个时间开始，一直延续到现在，并有可能持续下去。
如：She is known t o have been working on the problem for many years.2、不定式的语态当不定式的逻辑主语是其动作的承受者时，就用被动式。
如：He was seen to enter the hall.三、省to 的动词不定式1、情态动词（除ought 外，ought to）2、Would rather, had better.3、感官动词see, watch, look at, notice, observe, hear, listen to, smell, feel 等后作宾补，省略to.注意：在被动语态中to 不能省略掉。
如：I saw him dance. He was seen to dance. The boss made them work the wholenight.They were made to work the whole night.4、使役动词let, have, make.5、由and, or 和than 连接的两个不定式，第二个to 可以省去。
如：He wants to move toFrance and marry the girl.6、Help 可带to ,也可不带to, help sb. (to) do sth.7、Why…/Why not…8、But 和except 前是动词do 时，后面出现的动词用不带to 的动词不定式。
试比较：He wants to do nothing but go out.He wants to believe anything but to take the medicine.9、通常在discover, imagine, suppose, think, understand 等词后，可以省去to be .如：He is supposed to be nice. 他应该是个好人。