由于不定式具有名词、形容词和副词的特征，所以它在句中可作主语、表语、宾语、定语和状语。
1.作主语
To sweep the floor is my duty every day.
每天打扫地板是我的职责。
To see is to believe. =Seeing is believing.眼见为实。
it 作形式主语，当作主语的不定式短语太长时，常用it作形式主语而真正的主语(不定式短语)放在谓语之后。例如：
It is impossible to finish so much homework in a day.
在一天内完成这么多作业是不可能的。
2.作宾语
（1）. 动词+to do;
不定式可以直接跟在一个及物动词后面作宾语，这时句子有以下两个特点：
第一，句子的主语和不定式的逻辑主语是一致的，即两个动作都是由全句主语发出的。
第二，这时句子的谓语动词多是描写态度，不定式的动词则说明行为。
谓语动词主要有：afford，agree，aim，appear，arrange，ask，care，choose，claim，decide，demand，determine，endeavor，expect，fail，happen, help，hesitate，hope，learn，manage，offer，plan，prepare，pretend，proceed，promise，prove，refuse，resolve，seem，swear，tend，threaten，undertake，volunteer，vow，want，wish 等
I hope to see you again.
我希望再见到你。
This company refused to cooperate with us.
这家公司拒绝与我们合作。
He promised not to tell anyone about it.
他许诺不把这件事情告诉任何人。
有些动词既可以接不定式，也可以接动名词充当宾语，主要有：like, love, prefer, hate, bother, begin, start, cannot bear, continue, cease 等。举例：
Don't bother to get / getting dinner for me. 不必费心为我做晚饭了。
有些动词既可以接不定式，也可以接动名词充当宾语，但意思不同。
1). stop to do sth. 停下来去做某事；stop doing sth. 停止做某事。举例：
Stop to think about it for a while. 停下来考虑一会儿。
Stop talking, please. 别讲话了。
2). remember / forget to do sth. 记得/忘记要去做某事；remember / forget doing sth. 记得/忘记已经做过某事。举例：
I forgot to lock the door. 我忘记锁门了。
I forgot locking the door. 我忘记已经锁过门了。
3). try to do sth. 努力做某事；try doing sth. 尝试做某事。举例：
I'll try to do better next time. 我下次尽力做好。
He tried speaking English. 他尝试说英语。
4). go on to do sth. 接下来做某事；go on doing sth. 继续做某事。举例：
He went on to do the other excercises. 他接下来做其它的练习。
He went on doing the excercises. 他继续做练习。
5). regret to do sth. 很遗憾要去做某事；regret doing sth. 很后悔做了某事。举例：
I regret to do this, but I have no choice. 很遗憾，我只能这样做，没办法。
I regret telling you what I did. 真后悔告诉你我做了什么。
6). mean to do sth. 打算做某事；mean doing sth. 意味着做某事。举例：
I meant to call on you tomorrow. 我打算明天去看看你。
Missing the train means waiting for another hour. 错过这班车意味着再等一个小时。
（2）. 动词+疑问词+to do
在一些动词之后，可以在连接代词（what，who，which）或连接副词（how，when，where）及连词whether后面接一个带to的不定式，我们可以将这种结构看成是连接词引导的宾语从句的简略形式。事实上，这些连词都是由相应的疑问词转化而来的，其中whether对应一般疑问句。
I wonder who to invite(=who I should invite)我想知道该邀请谁。
I don't know whether to answer his letter.我不知道是否要给他回信。
Please show me how to do that. 请告诉我该怎么做。
（3）.动词+it+宾语补足语+to do
不定式(短语)作宾语，若不定式(短语)后面带有宾常将不定式放在宾补之后，而用it作形式宾语。例如：
She found it difficult to express what she wants to say.
她觉得要表达她想说的话很困难。
I think it hard to answer the question in one go.我觉得很难一下子回答这个问题。
这里it是形式宾语，它指不定式短语 to answer the question in one go这个真正的宾语。形容词hard是宾语补足语。
I think it quite important to learn English well. 我认为学好英语相当重要。
3.作状语
不定式(短语)作状语，可表示目的、原因、结果或件。例如：
The house is comfortable to (live)in(表结果).是一座住起来很舒服的房子。
1）. 动词不定式作目的状语。
To make himself heard， he raised his voice. 为了被听清楚，他提高了嗓门。（作目的状语）
He ran so fast as to catch the first bus. 他飞快地跑以便赶上第一班车。（作目的状语）
【注意】 为了强调，我们可以在不定式前加上in order或so as。如：
She had no option but to quit her job in order to take care of her child. 为了照顾孩子，她别无选择，只好辞职。
In order to catch the train， she hurried through her work. 为了赶火车，她匆匆做完了她的工作。
She got up earlier the next day so as to catch the first bus. 第二天她起得更早了，为的是赶上第一班车。
Mike is studying hard so as to keep up with others. 为了赶上别人，迈克正在努力学习。
2）. 动词不定式作结果状语。
（1） 动词不定式作结果状语的一般用法。如：
He woke to find himself famous. 他一觉醒来，发现自己成了名人。
As we all know， few live to be a hundred. 我们都知道，活到百岁的人不多。
（2） 在不定式前加only，表示一种“意外”的不好的结果。如：
He searched the room only to find nothing. 他翻遍了整个房间，没发现什么。
I hurried to the airport only to find the plane I was to take had just taken off. 我急急忙忙赶到机场，却发现我要乘坐的飞机已经起飞了。
（3） 英语中too+adj. / adv. +to do sth.来表示否定结果，意思为“太……以至不能……”，这时too后面的形容词或副词往往是表示性质或特征的词。如：
The girl is too young to go abroad， I think. 我认为这个女孩太年轻了，不能出国。
He is too young to go to school. 他太小了，不能上学。
【注意】 too+adj.+to do sth. 结构除表示否定意义外，还可以表示结果有肯定意思。这时too后面常常跟表示心情、态度或倾向性方面的形容词，主要有anxious， eager， easy， glad， happy， pleased， ready， willing等，此时too的意思是“十分;非常;很”等。如：
They were too anxious to leave. （=very anxious to leave） 他们急于要离开。
He is too glad to go there. （=very glad to go there） 他非常高兴去那儿。
I am too glad to get such a gift. 我很高兴能得到这样的一个礼物。
3）. 动词不定式作原因状语。
All of us are surprised to see his rapid progress. 看到他的进步，我们都很吃惊。
I am glad to see you. 见到你很高兴。
4). 动词不定式作条件状语。
How can you catch the train to start so late？ 要是这么晚动身，你怎么能赶得上火车呢？（表示条件）
He must be a fool to say so. 他那么说可能是个傻瓜。
You will do well to speak more carefully. 要是说得详细的话，你会做得很好的。
4.作定语不定式作定语时，须放在被修饰的名词或代词之后不定式与它修饰的词之间常为动宾关系，因此，如果不定式中的动词是不及物动词，它后面就应有必要的介词。例如：
Mary has two babies to (look after).玛丽要照看两个孩子。
(1).一般情况下，不定式与其所修饰的名词或代词之间存在逻辑上的动宾关系(即名词或代词是不定式动词的逻辑宾语，也就是动作的对象). 有时不定式中的动词后要加上相应的介词。
1) She has much homework to do.他有许多作业要做。(to do homework)
2) I have so many clothes to wash today. 我今天有许多衣服要洗。 (to wash clothes)
3) I 'd like something to eat. 我想要吃点东西。(to eat something)
4) We have no houses to live in. 我们没有房子住。(to live in houses)
5) Do you have any paper to write on? 你有用来写的纸吗？ (to write on paper)
6) He'd like to look for a restaurant to eat at.他想要找个饭店吃饭。
(to eat at a restaurant)
(2). 动词不定式所修饰的名词是time, way等时，不定式与这些名词呈现出动状关系；
不定式所修饰的名词是place时，不定式中的动词后面通常不带介词。
1)He needs time to do homework. 我需要时间来做作业。
2) He wants to know the best way to travel around the city.
他想知道全城观光的最好方法。
3）In fact,it is a wonderful place to live.
实际上，这是个居住的好地方。
5.作表语不定式作表语，一般紧跟系动词，如be，remain，appear, get，seem等后面，用来说明主语的内容、目的、性质、特征。例如：
What she wants to do seems to play basketball.她想要做的似乎就是打篮球。
His wish is to become an astronaut.他的愿望是成为一名宇航员。
The best way is to join an English club.最好的办法是加入一个英语俱乐部。
The first thing is to listen to the teacher carefully.首先是要认真听老师讲课。
What he hoped was to be admitted into the university.他希望能被大学录取。
注意：有些作表语的不定式，在结构上是主动的，但在意义上却是被动的。
She is to blame.
她应该受到责备。
The result is not long to see.结果不久就会看到。
6.作宾语/主语补足语
(1)在主动语态中作宾语补足语，例如：
The boss likes you to do it this way.
老板希望你这么干。
不定式可以在"主语+谓语+宾语+宾语补足语"句型中充当宾语补足语。在这一句型中，宾语是动词不定式的逻辑主语。
1）.在表示感觉的动词后作宾语补足语，并且不定式都不带to。
这类动词有：see, hear, feel, watch, notice, observe, look at, listen to等。
I heard them sing yesterday.昨天我听见他们唱歌了。
Did you see him go out你看见他出去了吗?
I felt something crawl up my leg.我感到有什么东西爬到我腿上了。
注意：转为被动语态时，原不带to的不定式要变成带to的不定式。notice和watch没有被动语态。
We saw the car stop.
The car was seen to stop.我们看见这辆车停了下来。
2）.在使役动词后作宾语补足语，不定式不带to。
这类动词有：make, let, have等。转为被动语态时，其后通常都用带to的不定式(have没有被动语态)。
有些动词跟不定式作宾语补足语时省去了to，这些动词有：
一感二听三让四观看。
一感：feel 二听：hear,listen to 三让：let,have,make 四观看：observe,see,watch.,look at
What would you have me do你要我做什么?
She made him give up smoking.她让他戒了烟。
Let him do whatever he wishes to do.他想干什么就让他干吧。
Though he had often made his little sister cry, today he was made to cry by his little sister. 虽然经常是他弄哭小妹妹，但今天他却被小妹妹弄哭了。
3）.在表示心理状态的动词后作宾语补足语。
这类动词有：consider, think, believe, discover, find, imagine, judge, suppose, prove等。这类动词后的不定式通常是"to be+形容词或名词"结构，think, consider, find后的to be常可省略。
We consider him (to be) a good teacher.我们认为他是一个好老师。
He proved that theory (to be) very important. 他证明那个理论是很重要的。
I thought her (to be) nice and honest the first time I met her.我第一次见到她的时候就认为她人很好，很诚实。
4）.在表示情感状态的动词后作宾语补足语。
这类动词有：love, like, prefer, hate, want, wish, expect等。
I'd prefer you to leave him alone.我希望你不要打扰他。
I don't want there to be any trouble.我不想有任何麻烦。
5）.动词advise, allow, ask, beg, command, tell, invite, force, oblige, get, help, encourage, persuade, permit, remind, request, order, warn, cause等后面，多接不定式短语作宾语补足语。
We don't allow such things to happen again. 我们不容许这种事情再发生。
Most of the parents agree to forbid their children to smoke.大多数父母都同意禁止自己的孩子抽烟。
She asked me to answer the phone in her absence.她请我在她不在的时候接电话。
Please remind me to leave her this note.请提醒我留给她这张纸条。
She requested him to go with her.她邀请他一同去。
注意：hope, demand, suggest等动词后面不能接不定式作宾语补足语。
【误】I hope you to give me a hand.
【正】I hope you can give me a hand.我希望你能帮我一把。
【正】I wish you to give me a hand.我希望你能帮我一把。
【误】He demanded me to be present at the meeting.
【正】He demanded that we should be present at the meeting.他要求我们出席会议。
【正】He required us to be present at the meeting.他要求我们出席会议。
【误】Mr Li suggested her not to go there alone.
【正】Mr Li suggested that she should not go there alone.李先生建议她不要独自去那里。
6）.动词不定式也可作一些短语动词的宾语补足语。
I'm waiting for James to arrive.我正等着詹姆斯的到来。
He's arranged for a car to pick them up at the station.他已安排了一辆汽车去车站接他们。
The UN has called on both sides to observe the ceasefire.联合国号召交战双方遵守停火协定。
必背： 这些带介词的短语动词有：
call on号召 arrange for 安排 ong for盼望 wait for等待 depend on 依靠 rely on 指望
(2)在被动语态的句子中作主语补足语，例如：
They are allowed to smoke here.他们被允许在这儿吸烟。.
动词不定式作主语补足语常见句型
常用不定式作主语补足语的句型有：be said （据说......）/ be reported（据报道......)/be thought(被认为......)/ be believed (被相信......）/be known( 被知......）/ be supposed （应该......）+ to do sth.
例如， He is said to speak five foreign languages.据说他会说五门外语。
The A.B.C. bank is reported in the local newpaper to have been robbed yesterday.
据当地的报纸报道，这家农业银行昨天被打劫了。
You are supposed to ask the maths teacher himself if you have any problem in study.
如果你在学习中有任何问题，你应该去问数学老师本人。
由动词不定式宾语补足语变主语补足语情况
在不定式作宾补的结构中 ，把它变成被动语态中,宾补就做成主补，例如，
The teacher tell us to read the short passage. (宾语补足语）
老师告知我们去阅读这篇短文。
We are told to read the short passage.(主语补足语）
The boss made the workers work day and night.(宾语补足语）
老板让工人们没日没夜的工作。
the workers was made to work day and night by the boss.(主语补足语）
He is said to be strict with his students. (一般时，一般情况）
据说他对他的学生很严格。
He is said to go to Shanghai next week. (一般时，将来意味）
据说他下个星期要去上海。
He is said to have been to Shanghai before. (完成时，表明事情已经完成）
据说他以前去过上海。
动词不定式作主语补足语，要把握好它，关键：一是注意熟悉经常使用动词不定作主语补足语的相关动词say,think ,believe,report,suppose等，二是注意时态，注意动词一般时与完成时的区别；三是注意，动词不定式作主语补足语一定要与被动语态连用。
(三)“疑问词十不定式”结构疑问词who, which, what, when, where, how, whether等后接不定式，构成不定式短语，在句中作主语、宾语、表语。例如：
They wondered whether to leave or not.
他们不知道是不是该离开。
(四)不定式中省去to的情况
1).在感官动词和使役动词之后。
感官动词：feel，hear，see，look at，watch，notice, observe，listen to使役动词：have，make，let,
I didn't hear anyone say anything about it.
我没有听到任何人谈过这件事。
2).以why not引导的疑问句中。例如：
You look so tired. Why not have a rest?
你看上去很累，干嘛不休息一下呢?
3).在would rather, than, rather than等结构中than之后用省略to的动词不定式。例如：
I'd rather walk than go by bus.
我宁愿走路也不愿乘车。
4.在cannot but, cannot choose but. cannot help but之后接省略to的不定式。例如：
I cannot but admire her courage. ,我只能钦佩她的勇气。
5.有两个以上的不定式并列。例如：
She promised to finish her homework and hand it in on time.
她答应完成作业并按时交上。