在语法中，动词不定式是指动词的一种不带词形变化从而不指示人称、数量、时态的形式。它之所以被叫做不定式，是因为动词不被限定，或者说不被词形变化所局限。下面小编告诉你八年级英语动词不定式的用法，大家一起来看看吧!
八年级英语动词不定式的用法:
不定式作表语
不定式可放在be动词后面，形成表语。例如:
My work is to clean the room every day.
His dream is to be a doctor.
不定式作定语
不定式做定语通常要放在被修饰的词后。例如:
I have a lot of work to do.
So he made some candles to give light.
不定式作状语
1)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……)
He ran so fast as to catch the first bus. 他飞快地跑以便赶上第一班车。
I come here only to say good-bye to you. 我来仅仅是向你告别。
2)作结果状语，表事先没有预料到的，要放在句子后面。
What have I said to make you angry.
He searched the room only to find nothing.
3) 表原因
I’m glad to see you.
典型例题
The chair looks rather hard, but in fact it is very comfortable to ___.
A. sit B. sit on C. be seat D. be sat on
答案:B. 如果不定式为不及物动词，其后应有必要的介词。当动词与介词连用时，常位于"形容词+动词不定式"结构的末尾。
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不定式主语
1) It’s easy (for me) to do that.我做这事太容易了
easy,difficult,hard,important,possible,impossible,comfortable,necessary,better;
the first,the next,the last,the best,too much,too little,not enough
It’s so nice to hear your voice.
听到你的声音真高兴。
It’s necessary for you to lock the car when you do not use it.
当你不用车的时候，锁车是有必要的。
2) It’s very kind of you to help us. 他帮助我们，他真好。
Kind,nice,stupid,rude,clever,foolish,thoughtful,thoughtless,brave,considerate(考虑周到的),silly,selfish(自私的)
例句:
It was silly of us to believe him. 我们真愚蠢，竟然相信了他。
It seemed selfish of him not to give them anything. 他不给他们任何东西，这显得太自私了。
注意:1) 其他系动词如，look，appear等也可用于此句型
2) 不定式作为句子成分时，动词用单数形式。
3) 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
(对)To see is to believe. 百闻不如一见。
(错)It is to believe to see.
不定式作补语
1) 动词+宾语+不定式(to do)
advise,allow,appoint,believe,cause,challenge,command,compel,consider,declare,drive,enable,encourage,find,forbid,force,guess,hire,imagine,impel,induce
例句:
a. Father will not allow us to play on the street.
父亲不让我们在街上玩耍。
b. We believe him to be guilty.
我们相信他是有罪的。
Find 的特殊用法:
Find 后可用分词做宾补，或先加形式宾语，再加形容词，最后加带to 的动词不定式。find后也可带一个从句。此类动词还有get，have。
I found him lying on the ground.
I found it important to learn.
I found that to learn English is important.
典型例题:
The next morning she found the man ___ in bed，dead.
A. lying B. lie C. lay D. laying
答案:的宾语后面，用分词或分词短语，起宾语补足语作用。现在分词表达主动，也表达正在进行，过去分词表达被动。
2) to + be 的不定式结构，作补语的动词。
Acknowledge, believe, consider, think, declare(声称), discover, fancy(设想), feel find, guess, judge, imagine, know, prove, see(理解), show, suppose, take(以为), understand
We consider Tom to be one of the best students in our class.
我们认为汤姆是班上最好的学生之一。
典型例题
Charles Babbage is generally considered ___ the first computer.
A. to invent B. inventing C. to have invented D. having invented
答案:A. 由consider to do sth. 排除B、D。. 此句只说明发明这一个事实，不定式后用原形即可。而C为现在完成时，发明为点动词一般不用完成时，且此处也不强调对现在的影响，因此不选C。
3) to be +形容词
Seem, appear,be said,be supposed,be believed,be thought,be known,be reported,hope, wish, desire,want,plan, expect,mean…
The book is believed to be uninteresting.
人们认为这本书没什么意思。
4) there be+不定式
believe, expect, intend, like, love, mean, prefer, want, wish, undrstand
We didn’t expect there to be so many people there.我们没料到会有那么多人在哪里。
注意 : 有些动词需用as 短语做补语，如regard, think believe, take, consider.
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。
不定式作宾语
1) 动词+ 不定式
afford aim appear agree arrange ask be decide bother care choose come dare demand desire determine expect elect endeavor hope fail happen help hesitate learn long mean manage offer ought plan prepare pretend promise refuse seem tend wait wish undertake
举例:
The driver failed to see the other car in time.
司机没能及时看见另一辆车。
I happen to know the answer to your question.
我碰巧知道你那道问题的答案。
2)动词+不定式 ; 动词+宾语+不定式
ask, beg, choose, expect , hate, help intend like, love, need prefer, prepare, promise, want, wish…
I like to keep everything tidy. 我喜欢每件东西都保持整洁。
I like you to keep everything tidy. 我喜欢你年使每件东西都保持整洁。
I want to speak to Tom. 我想和汤姆谈话。
I want you to speak to Tom. 我想让你和汤姆谈话。
3) 动词+疑问词+ to
decide, know, consider forget, learn, remember, show, understand, see, wonder, hear, find out, explain, tell
Please show us how to do that. 请演示给我们如何去做。
There are so many kinds of tape-recorders on sale that I can’t make up my mind which to buy.有这么多的录音机，我都拿不定主意买哪一种。
注意:疑问词带不定式在句中作成分时，谓语动词用单数。
The question is how to put it into practice.
问题是怎样把它付诸实施。
两种不同性质的“be +不定式”结构:
【问题】以下两组句子，be之后都是动词不定式，请问它们有何区别?
第一组:
Her job is to look after the children. 她的工作是照顾这些孩子。
My aim is to start up my own company. 我的目标是开办一家我自己的公司。
His ambition was to meet his favourite pop star face to face. 他的心愿是，要面对面地见到他所喜爱的流行歌星。
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第二组:
He is to stay here until we return. 在我们回来之前他必须呆在这里。
Such people are to be pitied. 这种人应受到怜悯。
If we are to succeed, we must rely on the masses. 我们要成功必须依靠群众。
英语中，“不定式作表语”和表将来的“be +不定式”结构，是两个不同的概念。第一组是“不定式作表语”; 第一组是表将来的“be +不定式”结构。我们用最简单的方法做一区别，请看这两个句子:
Her job is to look after the children. 她的工作是照料这些孩子。
She is to look after the children. 她将要照料这些孩子。(表将来，可能是安排的任务)
这两句话只是主语不同，其他部分看起来完全一样，意思却大不相同，这是为什么?
它们看起来太相似了，以至于有时难以区别，这确实是一个很有意思的问题。
Her job is to look after the children. 意为 “她的工作是照料孩子的。” 本句中，is 是系动词，to look after children是不定式作表语。主语和表语可以互换而意思保持不变:To look after the children is her job. (照料孩子是她的工作。)
She is to look after the children. 意为“她将要照料孩子。”本句中，is是助动词，跟不定式to look一起，表示将来。因为不是主系表关系，所以is前后两部分不能颠倒，不可以说:To look after the children is she.
问题中的第一组都可以“主表颠倒”，变成:
To look after the children is her work. 照顾这些孩子是她的工作。
To start up my own company is my aim. 开办一家我自己的公司是我的目标。
To meet his favourite pop star face to face was her ambition. 面对面地见到他所喜爱的流行歌星是他的心愿。
第二组则不可以颠倒，因为颠倒后句意逻辑不成立。
【判断规则】A代表“主语”，B代表“不定式”:
“主系表结构”:A is B. 可以说:B is A. (is意为“是”)
“将来时结构”:A is B. 不可以说:B is A. (is to do意为“将要做”)
习惯上接不定式作宾语的动词:
英语中有些动词后接另一个动词作宾语时，用作宾语的动词习惯要用不定式，而不用动名词，这类动词主要afford(负担得起), agree(同意), apply(申请), arrange(安排), ask(要求), care(想要), choose(决定), decide(决定), demand(要求), determine(决心), expect(期待), help(帮助), hope(希望), manage(设法), offer(主动提出), plan(计划), prepare(准备), pretend(假装), promise(答应), refuse(拒绝), want(想要), wish(希望)等。如:
Less and less people can afford to go abroad for their holidays. 花得起钱去国外度假的人越来越少了。
He agreed to help but backed out when he found how difficult it was. 他本来同意给予帮助的，但在发现事情有多么困难之后就打退堂鼓了。
Bill applied to join the police last week, but he wasn’t tall enough. 比尔上星期申请参加警察部队，但是他不够高。
We arranged to meet at the station but she didn’t turn up. 我们商量好在车站见面，可是她没有露面。
There have been fewer problems for travellers who chose to fly. 对于选择乘飞机旅游的人来说问题更少。
He decided to put an advertisement in the newspapers. 他决定在报上登一份广告。
He was very ill for a time, but he managed to pull through. 有一段时间他病得很重，但他最后痊愈了。
She pretended to be angry, but she was not. 她假装生气，却并未生气。
不定式和现在分词作结果状语的区别:
不定式和现在分词均可用结果状语，但两者用法有区别:
(1) 现在分词用作结果状语
通常表示一种自然的结果，即属预料之中的事。如:
He died, leaving his wife with five children. 他死了，留下他妻子和五个儿子。
The child fell, striking his head against the door and cutting it. 那孩子跌倒了，头碰在门上磕破了。
(2) 不定式用作结果状语
主要表示没有预料到的情况或结果，即属预料之外的事。该用法通常与副词only连用。如:
He lifted a rock only to drop it on his own feet. 他搬起石头砸了自己的脚。
He hurried to the house only to find that it was empty. 他急忙赶到那所房子，却发现空无一人。
She got there only to be told that they had gone on holiday. 她到达那里，被告知他们度假去了。
注意:并非所有表示结果的不定式都含有这种“意外”的效果。如:
What have I done to offend you. 我干什么惹你生气了?
The tea is too hot to drink. 茶太热，不能喝。
