1.作主语
（1）不定式在句子中可以作主语，谓语动词用单数。
To study hard is necessary.努力学习是有必要的。
To get up early is a good habit.早起是个好习惯。
（2）不定式作主语的句子变被动语态时，必须用形式主语。
It was decided to set up a new football club in our school.
已经决定在我们学校建立一个新的足球俱乐部。
It is wanted to make big money by most people.许多人都想挣大钱。
（3）形式主语：常用it来作形式主语，代替不定式或不定式短语。
It is+名词+to do sth.做……是……
It is+形容词+to do sth.做……是……
It is+形容词+for sb.to do sth.对某人来说，做……是……
It is+形容词+of sb.to do sth.某人做…… 是……
It’s like sb.to do sth.像某人做某事的风格
It seems/appears+形容词+to do sth.做……看起来……
It takes sb.some time to do sth.做……花了某人多少时间
It’s up to sb.to do sth.由……决定
It is our duty to serve the people.
为人民服务是我们的责任。（真正主语to serve ...）
It is not easy to learn a foreign language.
学会一门外语是不容易的。（真正主语to learn ...）
It is difficult to do the job.做那项工作很困难。（真正主语to do ...）
It’s very necessary for us to practise listening and speaking.
我们很有必要练习听和说。（真正主语for us to practice ...）
It is so nice of you to come to see me.
你来看我太好了。（真正主语of you to come ...）
It’s like him to leave the work to others.（真正主语to leave ...）
他就是这样的一个人，总是把工作推给别人。
It seems good to have a drink now.
现在喝点什么倒还不错。（真正主语to have ...）
It took more than 100，000 slaves twenty years to build the Great Pyramid.
十万多个奴隶花了20年的时间建成了那座大金字塔。（真正主语to build ...）
It is up to parents to teach their children manners.
该由父母教孩子礼貌。（真正主语to teach ...）
2.作表语
（1）表示目的打算及具体内容。
His wish was to become a skilful worker.他的愿望是要成为一名技术熟练的工人。
What he hoped was to be admitted into the university.他的希望就是能被大学录取。
（2）不定式被动形式作表语，相当于can/could，should，ought to，must等意义。
You are to be rewarded.你应该受到奖励。（should）
It’s nowhere to be found.哪儿也找不到它。（can’t be）
These books are not to be sold.这些书不应该卖掉。（ought not to be）
（3）表示“同意、安排、命令、决定、劝告、意愿、禁止”等。
They are to marry next week.他们将在下周结婚。（安排）
Children are not to smoke.儿童不准吸烟。（禁止）
You must be persistent if you are to succeed.想要成功，就必须有毅力。（愿望）
（4）seem，appear，prove，turn out等后接to be不定式时，可省略to be。
He seems （to be）ill.他似乎生病了。
The man turned out （to be）an cheater.这人竟是个骗子。
She proved （to be）a very strict teacher.她被证明是一位非常严格的老师。
He appeared （to be）about fifty years of age.他看起来大约50岁。
【提示】 主语中含有do的某种形式时，作表语的不定式可省略to。
What he wanted to do was （to）help the old man.他想做的是要帮一下这个老人。
3.作宾语
（1）作及物动词的宾语
常用不定式作宾语的动词：
afford 负担起 agree 同意 arrange 安排 attempt 打算
ask 要求 care 想要 choose 选择 dare 敢于
decide 决定 demand 要求 determine 决心 expect 期待
fail 未能 hesitate 犹豫 hope 希望 intend 想要
learn 学习 long 渴望 manage 设法 offer 主动
plan 计划 prepare 准备 pretend 假装 promise 答应
refuse 拒绝 seek 试图 want 想要 wish 希望
We have decided not to go.我们决定不去了。
He has promised to get me a position.他答应给我安排一个职位。
We have decided to experiment with a new method.我们决定用新方法进行试验。
We want to hear all about his adventure.我们想知道他奇遇的一切情况。
（2）“疑问词+不定式”作动词宾语
I don’t know what to do next.我不知道下一步该做什么。
I asked how to solve the problem.我问如何解决这一问题。
He forgot where to buy such a book.他忘了去哪儿买这样一本书。
He has to learn how to collect water，hunt for food，and make fire.
他得学会收集淡水、寻找食物、生火什么的。
（3）作介词的宾语
I have no idea of who to do it.我不知道谁去做此事。
They were concerned with what to deal with the problem.
他们关心的是如何解决问题。
Your success will largely depend upon how to do it.
你成功与否将主要取决于你怎样做。
They are discussing about where to give the lecture.他们正在讨论在哪里举行讲座。
【提示】 介词的宾语主要由“疑问词+不定式”短语充当。
4.作宾语补足语
（1）不省略to的动词不定式作宾语补足语
常用“vt.+sb.to do sth.” 句式的动词：
advise 劝告 allow 允许 ask 请求 bear 容忍
beg 乞求 cause 导致 command 命令 direct 指导
drive 驱使 elect 推举 enable 使某人能够 encourage 鼓励
expect 期望 forbid 禁止 force 强迫 get 让某人去
hate 讨厌 help 帮助 intend 打算 invite 邀请
like 喜欢 mean 打算 need 需要 oblige 迫使
order 命令 permit 允许 persuade 说服 prefer 宁愿
recommend 推荐 remind 提醒 request 要求 require 要求
teach 教育 tell 告诉 train 训练 trouble 麻烦
want 想要 warn 警告 wish 希望
The teacher asked us to finish our homework.老师叫我们完成家庭作业。
She wanted him to sing for her friends.她想让他为她的朋友唱歌。
The policeman told the boys not to play in the street.警察告诉孩子们别在街上玩耍。
Please remind me to post this letter.请提醒我寄这封信。
【比较】
①I don’t allow/permit him to swim.我不允许他游泳。（不定式作宾补）
I don’t allow/permit swimming here.我不许在此地游泳。（动名词作宾语）
②I advise you to take medicine.我建议你吃药。（不定式作宾补）
I advise taking medicine.我建议吃药。（动名词作宾语）
（2）“to be不定式”作宾语补足语
常用“vt.+sb.+to be ...”句式的动词：
acknowledge 承认 believe 相信 consider 认为 declare 宣称
discover 发现 feel 感觉 find 找到 guess 猜想
imagine 想象 judge 判断 know 知道 prove 证明
suppose 猜想 take 以为 think 认为 understand 了解
I consider him to be the cleverest.我认为他是最聪明的。
We discovered him to be a cheat.我们发现他是一个骗子。
I find English to be very easy.我发现英语很容易。
I supposed him to be away from home.我想他不在家。
【提示】
①这类句式后的不定式用进行式或完成式时，to be不能省略。
They believe him to be writing a new book.他们认为他正在写一本新书。
I consider the work to have been finished.我估计活已干完了。
②这类句式的被动结构为“主语+is done+to do/be doing/have done”。
The film is believed to be the best this year.这部电影被认为是今年最好的。
He is believed to be writing a new book.人们认为他正在写一本新书。
They are supposed to have finished.他们应该完成了。
③作短语动词后的宾语补足语
appeal to sb.to do sth.呼吁某人做某事 arrange for sb.to do sth.安排某人做某事
call on sb.to do sth.号召某人做某事 care for sb.to do sth.愿意某人做某事
count on sb.to do sth.指望某人做某事 depend on sb.to do sth.依靠某人做某事
long for sb.to do sth.盼望某人做某事 rely on sb.to do sth.依靠某人做某事
wait for sb.to do sth.等待某人做某事 would like sb.to do sth.希望某人去做某事
The government is appealing to everyone to save water.政府呼吁人人节约用水。
The crocodile waited for the monkey to come down again.鳄鱼等着猴子再下来。
The chairman called on Mr Brown to speak.主席请布朗先生讲话。
（4）省略to的不定式作宾语补足语
see 看到 watch 观看 notice 注意 observe 观察 look at 看着
hear 听到 listen to 听着 feel 感觉 make 使 let 让 have 使
You must watch me carefully do everything.你必须仔细观察我所做的一切。
I often hear him sing this song.我经常听到他唱这首歌。
I felt someone open my door.我感觉有人开了我的门。
It will make the cabbage grow big and strong.它将使白菜长得又大又壮。
【提示】 这类句式变为被动语态时，不定式符号to 要补出。
He was seen to write a letter.有人看见他写了一封信。
（5）it作形式宾语
They found it impossible to get everything ready in time.
他们发现不可能把一切按时准备好。（真正宾语to get ...）
She thought it unnecessary to argue with him.
她觉得没有必要和他辩论。（真正宾语to argue ...）
Do you consider it better not to go？ 你觉得不去更好吗？（真正宾语not to go）
5.作定语
（1）主谓关系
He was the best man to do the job.他是做这工作的最佳人选。
Gina is the first to know my e-mail address.吉娜是第一个知道我电邮地址的。
You are the right person to do the job.你是做这项工作合适的人选。
She was the only one to survive in the air crash.她是空难中唯一幸存的人。
【提示】 the last ...to do sth.意为“最不可能的……”。
He is the last man I want to see.他是我最不想见的人。
She is the last woman in the world to be a farmer’s wife.她最不适宜作农夫的妻子。
（2）动宾关系
I have many letters to write.我有许多信要写。
I have some clothes to wash.我有一些衣服要洗。
It was a game to remember.那是一场令人难忘的球赛。
【提示】
①在“动宾关系”中，不定式作定语要用及物动词的主动形式。
I have a lot of work to do today.我今天有很多工作要（自己）做。
②如果动词是不及物动词，其后要加必要的介词。
Give me a pen to write with.给我一只钢笔写字用。
Maria is a pleasant person to work with.和玛丽亚一起工作真愉快。
He has no house to live in.他没有房子住。
I have enough money to buy the book with.我有足够的钱买这本书。
I lit a candle to read by.我点起蜡烛来看书。
（3）偏正关系：不定式常用于某些名词后作定语。
ability 能力 ambition 志向 anxiety 焦虑 attempt 企图
campaign 运动 chance 机会 courage 勇气 decision 决定
desire 欲望 determination 决心 effort 努力 failure 失败
refusal 拒绝 promise 许诺 tendency 趋势
I had a dream to fly in the sky like a bird.我做了一个像鸟儿一样在天上飞的梦。
I have a wish to go to college.我有一个上大学的愿望。
They made a plan to study English.他们制订了一个学习英语的计划。
Everyone has the right to speak at the meeting.人人都有在会议上讲话的权利。
【提示】
①在idea，habit，method，purpose等词后只用“of doing ...”作定语。
They got the happy idea of climbing the hill.他们想到爬山的好主意。
I am in the habit of rising early.我习惯早起。
②在way，pleasure，time，chance，opportunity，plan，power等词后可用“to do sth.或of doing sth.”作定语。
Do you know the way of mastering English？ 你知道精通英语之道吗？
Do you know the way to master English？
【比较】
①She was on her way to catch the school bus.
她在赶搭校车的途中。（on one’s way to do ...）
②He is on his way to becoming a journalist.
他将会成为一名新闻记者。（on one’s way to do ...）
6.作状语
（1）作目的状语
We started early to avoid being late.我们一早就动身免得迟到。
I came to hear the report.我是来听报告的。
【辨析】我一早动身以便及时到达那里。
【正】To get there in time I started early.（to do置于句首，作目的状语）
【正】I started early to get there in time.（to do置于句末，作目的状语）
【正】In order to get there in time I started early.（in order to目的状语，置于句首）
【正】I started early in order to get there in time.（in order to目的状语，置于句末）
【正】I started early in order that I could get there in time.（in order that引导目的状语从句）
【误】So as to get there in time I started early.（so as to 目的状语不能置于句首）
【正】I started early so as to get there in time.（so as to 目的状语只能置于句末）
【正】I started early so that I could get there in time.（so that引导目的状语从句）
（2）作结果状语
He grew up to be a famous scholar.他长大以后成了有名的学者。
I opened the door to find the room empty.我打开门，结果发现房间是空的。
He woke up to find everyone gone.他醒来发现人都走了。
不定式作结果状语的常用句式：
（1）“only to do ...”表示“出乎意料的结果”；
（2）“never to do ...” 表示“再也没有……的结果”；
（3）“such/so ...as to do ...”表示“如此……以至于”；
（4）“adj./adv.+enough to do ...”表示“足以……”；
（5）“too ...to do ...”通常表示否定意义，意为“太……以致不能”。
He worked hard only to fail.他努力工作结果却只有失败。
They parted，never to see each other again.他们分手了，从此没有再见面。
He isn’t old enough to join the army.他还不到参军的年龄。
He runs fast enough to catch the bus.他跑得够快，足以赶上公共汽车。
He is too small to join the Youth League.他的年龄太小，不能加入共青团。
The water is too dirty to drink.水太脏，不能喝。
He was so kind as to be helpful to others.他非常友好，总是帮助别人。
His story is such a sad story as to arouse our sympathy.
他的经历这么悲惨，引起了我们的同情。
【提示】 “so ...as to do”句式中so接形容词或副词；“such ...as to do”句式中such接名词。
（3）作原因状语
The children jumped with joy to hear of the arrival of Father Christmas.
孩子们听说圣诞老人来了就高兴得跳了起来。
We were pleased to hear the good news.听到这个好消息，我们很高兴。（4）不定式作插入语
To tell the truth，I have no money with me.坦白地说，我身上没带钱。
Strange to say，his hair turned white during the night.
说也奇怪，他的头发一夜之间变白了。