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.
(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.
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
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.
I find it necessary to learn a foreign language.
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
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.
I often help him (to)clean the room.
I helped him (to) find his things.
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)
Do you have anything else to say?
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)
5、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。
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.
He hurried to the post office only to find it was closed.
He searched the room only to find nothing.
We were very excited to hear the news.
I’m glad to see you.
To turn to the left , you could find a post office.
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.
如：To see is to believe. (眼见为实)
To tell you the truth, I don’t agree with you.
To make matters worse, it began to rain.
He seems to know this. I hope to see you again.
I’m sorry to have given you so much trouble.
He seems to have caught a cold.
He seems to be eating something.
She is known to have been working on the problem for many years.
He was seen to enter the hall. He asked to be sent to work in Tibet.
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 whole night.
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 to France and marry the girl.
Help 可带 to ,也可不带to, help sb. (to) do sth.
6、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.
7、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。
——I usually go there by train.
——Why not ______ by boat for a change?
A to try going B trying to go C to try and go D try going
Paul doesn’t have to be made ______ . He always works hard.
A learn B to learn C learned D learning
(四)、 动词不定式的否定式。如：Tell him not to shut the window. She pretended not to see me when I passed by.
Mrs. Smith warned her daughter ______ after drinking.
A never to drive B to never drive C never driving D never drive
The boy wanted to ride his bicycle in the street, but his mother told him ______.
A not to B not to do C not do it D do not do
The patient was warned ______ food before the operation.
A to eat no B eating not C not to eat D not eating