(三)作定语。
不定式短语作定语时，一般放在被修饰的词之后，有些名词的后面常用不定式作定语。常见的有：
ability能力，agreement一致，协议，ambition抱负、野心，anxiety焦急，attempt试图，campaign战役，chance机会，claim声称，courage勇气，decision决定，determination决心，drive努力，eagerness殷切的心情，effort努力，failure失败，hope希望，intention目的意图，motive force动力，movement运动、协议，need需要，opportunity机会，plan计划，promise诺言，readiness乐意，refusal拒绝，reluctance勉强不愿，resolution决心，tendency倾向，right权力，struggle斗争，threat威胁，time时候，way方法，willingness愿意，wish希望。
与这些词相对应的形容词或动词常接动词不定式。如ability →be able to, refusal →refuse to等。
作定语时不定式表示将要发生的动作。
There is not any one to save him. 没有任何能救他的人。
He’s a not a man to tell lies. 他不是那种撒谎的人。
He’s a man to be trusted. 他是个可以信赖的人。
He needs love to strengthen his broken heart. 他需要爱来给他那破碎的心以力量。
I don’t understand his idea to send his son to the Sahara. 我不明白他把儿子送到撒哈拉沙漠的想法。
I need to buy a box to hold my CDs. 我需要买个装CD片的盒子。
This visit is a good opportunity to learn Chinese. 这次访问是学习中文的好机会。
No investigation, no right to speak. 没有调查就没有发言权。
Not to speak much is the best way to avoid trouble. 不多言是避免麻烦的最好办法。
This is a good way to make money. 这是个赚钱的好方法。
We were given some bread to eat. 给了我们一些面包吃。
I’ve a lot of things to take care of in the office today. 今天我办公室里由于很多事情要处理。
Have you got a key to unlock this door? 你有开这个门的钥匙吗?
He needs a place to live in. 他需要有个住的地方。
This is not a pen to write with but to draw with. 这不是写字而是画画用的笔。
I have not a friend to talk with. 我没有能谈心的朋友。
作定语时，也可以给不定式加上主语。例如：
The order for the army to start has been given. 已经发出了让部队开始的命令。
The power for science to serve mankind is limitless. 科学为人类服务的力量是无穷的。
The letter for you to answer is placed on your desk.. 要你回的那封信放在你办公桌上。
(四)作表语，放在系动词之后。例如：
To gamble is to commit suicide, but not to get rich. 赌博就是自杀，而不是致富。
To criticize others is to make enemies. 批评别人就是树敌。
To do everything is to do nothing. 什么都做等于什么也做不成。
Perhaps the most popular way of relaxing is to participate in sports. 也许最受欢迎的放松方式就是参加体育活动。
This matter is for you to consider. 这个问题是要你考虑的。
These apples are for the guests to eat. 这些苹果是给客人们吃的。
My hope is for all members to come together once a year. 我的希望是所有成员每年聚会一次。
My advice is for you to apologize to her. 我的建议是你向她道歉。
The question is where to go for help. 问题是到哪里求助。
To respect others is to be respected. 尊重别人就是受到别人的尊重。
To see her is to love her. 见到她就会爱上她。
(五)不定式短语和疑问词连用 (who, whom, whose, what, which, when, where, how,还包括whether)
(1)做主语。例如：
Who to turn to is what she wants to know. 找谁帮忙是她想知道的。
It’s problem what to do. 干什么还是个问题。
How to be wealthy, healthy and happy will always remain a question mark. 怎样才能富有，健康和幸福将永远是个问号。
(2)做表语。例如：
The question is which to take first. 问题是先拿哪一个。
What remains for you to explain is how to put the plan into practice and whose help to seek. 剩下需要你解释的是怎样实施计划以及寻求谁的帮助。
The question is which of the methods to adopt. 问题是采取这些方法中的哪一个。
(3)作宾语，常用这一结构作宾语的动词有ask, consider, decide discuss, explain, forget, find out, guess, inquire, know, learn, remember, see(=understand),show,settle(=decide),tell(=know),think,understand,wonder等。
I’ve found out where to buy them cheaply. 我发现了在哪里买这些东西便宜。
Do you remember when totch off the machine? 你记得什么时候关掉机器吗?
I can’t tell what to do . 我说不好怎么办。
How can you tell which button to press? 你怎么知道按哪个按钮?
You know very well when to say what and how to say it.
你很清楚什么时候说什么话以及怎样说。
He can’t decide what time to leave or whether to leave at all.他不能决定何时离开或者到底是否离开。
有些双宾语及物动词，也可用这个结构做直接宾语。常用的有tell, inform, show, advise, ask, teach,等。如：
I showed her how to use the remote control. 我向她演示怎样使用遥控器。
The salesman told him whichtch to turn on first. 售货员告诉他先开哪个开关。
Will you advise me which to buy? 你建议我买哪个?
Please inform me where to get the tickets. 请告诉我到哪里去弄票。
(4)做定语。例如：
I must find a pen with which to fill in this form(=to fill in this form with). 我必须找支钢笔填表用。
He offered me some hot coffee with which to refresh me spirit (=to refresh me spirit with).
他给我提供了点热咖啡提提神。
Nobel began to seek ways in which to make the explosive safer to handle.诺贝尔开始寻找那种炸药操作起来更安全的方法。
(六)作状语。不定式短语作状语，表示目的、原因、结果等。位置一般在句尾或句首。
(1)表示目的。如：
We waved flags, beat drums and blew bugles to encourage the players. 我们摇旗擂鼓吹号，来鼓励运动员。
They went to the U.S.A. not to study English but to study law and business.
他们去了美国，不是为了学英语，而是为了学法律和商务。
为了强调，有时用in order to (为了)或so as to (以便)加动词原形，放在句首或句尾均可。例如：
In order to improve my English, I read China Daily every day. 为了提高英语水平，我每天都看《中国日报》。
He took a taxi so as not to miss the train. 为了不误火车，他坐了出租车。
有些不定式习惯用语放在句首或句尾，作独立成分。例如：
To be frank(with you), this is not satisfactory. 老实(和你)说，这不令人满意。
To be fair, she is an honest girl. 说句公道话，她是一个诚实的姑娘。
To tell you the truth, you are wrong.. 实话告诉你，你错了。
这样的短语还有to tell you the truth实话告诉你，to conclude 最后， to begin with首先，to cut a long story
short 简而言之，to go back to the point回到正题，not to make too much of it 别小题大做，to put it another way 换句话说，to hear him talk听他说话的口气，等等。
(2)表示原因。例如：
To hear him talk in that manner, you would think him a foreigner. 他这样说话，你会以为他是外国人。The old father smiled to know his success. 知道他成功的消息，那位老父亲笑了。
不定式短语可以和表示情绪的形容词和过去分词连用，有时说明产生这种情绪的原因，有时不是说明原因，而是和形容词一起说明主语的情况。
常见的这类形容词和过去分词有：
able有能力的，afraid 害怕的，amazed 感到惊异的，angry生气的，anxious 急于的，astonished 感到惊讶的， careful小心的， certain有把握的，clever聪明的， comfortable舒服的，cruel残忍的，deep深的，delighted 高兴的， difficult有困难的， disappointed感到失望的，determined 有决心的，eager 急于的， easy容易的，fit 合适的，foolish 愚蠢的，fortunate 幸运的， free自由的，frightened感到害怕的，glad高兴的，good好的， happy幸福的，hard艰苦的，lucky 幸运的，overjoyed 高兴的， pleased高兴的，possible 可能的，proud 自豪的，ready 准备好的，relieved感到轻松的，right正确的，sad悲伤的，shocked感到惊讶的， sorry难过的，sure 肯定的，surprised感到惊讶的， unable没能力的，useless没用的，willing 愿意的，worthy 值得的，wrong错误的等。例如：
He’ll be angry to hear these words. 听到这些话他会生气的。
You’re lucky to have me as your teacher. 有我当你们老师，你们很幸运。
He feels proud to be on the school football team. 作为校足球队队员他感到很自豪。
Be careful not to catch cold. 小心别感冒。
He’s anxious to go back straight to London. 他急于直接回伦敦。
They’ll be ready to leave in about an hour. 再有大约一小时他们就准备好了。
The new manager is easy to get along with . 新经理很容易相处。
This river is broad, deep and good tom in. 这条河宽且深，是游泳的好地方。
Your sofa is comfortable to sit in. 你的沙发坐上去很舒服。
(3)不定式做结果状语只限于下面几个词：learn得知，find 发现，see 看见，hear 听见， to be told被告知，make 使得 等。only+不定式常表示出乎意料的结果。例如：
He hurried to the airport to learn that the flight had already taken off. 他急忙赶到机场，却得知飞机已经起飞了。
He returned home from his holiday only to find that his house had been broken into. 他休假回家，却发现他的房子被破门而入了。
(4) 不定式还可以与“be said, be reported,…”等连用构成复合谓语。这一结构可变成“It’s said, It’s reported,…”(见被动语态部分)。如：
An explosion is reported to have happened, and three are known to have been killed.
= It’s reported that an explosion has happened and it’s known that three have been killed.
据报道发生了一起爆炸，已知三人被炸死。
(七)不定式的时态。动词不定式一般有三种时态形式，即一般式、进行式和完成式。
(1) 一般式(to+动词原形)：表示与主句谓语动词动作同时(或几乎同时)发生或在主句谓语动词之后发生的事情。例如：
We’re happy to have you on our side. 有你在我们这边我们很高兴。
I saw him go out. 我看见他出去了。( 不定式go out 与 see同时发生。)
They invited me to have dinner with them. 他们邀请我和他们一起吃晚饭。
I hope to see you again. 我希望再见到你(to see发生在hope之后)
(2) 进行式(to be+现在分词)：表示主句谓语的动作发生时，不定式动作正在进行。例如：
She’s said /believed to be living nearby. 据说/据信她就住在附近。
When he came in, I happened to be sleeping in bed. 他进来时，我碰巧正在床上睡觉。
They seem/appear to be enjoying themselves. 看上去他们似乎很快活。
He pretended to be doing his homework when he heard his father’s sound. 他听见父亲的声音，假装正在做作业。
The president was reported to be visiting the hospital. 据报道总统正在访问那家医院。
(3)完成式(to have+过去分词)：表示在主句谓语的动作之前发生的事情。
如：I’m glad to have met your parents here. 我很高兴在这儿见到了你父母。(已见过)
I’m sorry to have kept him waiting for me so long. 我很抱歉让他等了我这样久。(已经等过了)
(八)不定式的被动语态。当不定式的逻辑主语是这个不定式所表示的动作的承受者时，不定式一般要用被动形式。
不定式的被动形式有两种：一般式 to be+过去分词;完成式 to have been+过去分词。例如：
You’re lucky to have been accepted . 你很幸运已经被接受了。
These criminals are to be hanged . 这些罪犯是将被绞死的。
We’re glad to have been invited. 受到邀请我们很高兴。
What’s to be done next? 下一步做什么?
It is an honor for me to be asked to speak here. 我很荣幸应邀在这里讲话。
为了避免重复，不定式可省略，但to有时保留，有时不保留。例如：
You will make it if you try (to). 如果你努力，你会成功的。
George says he is going to leave Shanghai, but I don’t think he really wants to. George说要离开上海，但是我不认为他真的愿意走。
Some of them retired, and others were ready to. 一些人退休了，还有的准备退。
He always speaks faster than he needs to. 他说话总是没必要的快。
I’d like to do it now, but I haven’t got the time (to). 我倒是想现在干，但没时间。
“Would you go there with me?” “I’m glad to.” “你愿意和我一起去吗?”“愿意。”
“Did you pass the exam?” “No, I tried (to), but I failed.” “你考试及格了吗?”“没有，我努力了，但没成功。
“Do you want to sing them an English song?” “I prefer not to.” “你愿意为他们唱首英文歌吗?”“我不想唱。”
You’d better sing an English song if they ask you (to) again.如果他们再要求你，你最后唱一首英文歌。
后面常省略不定式而保留to的结构，还有：
used to 常常 be going to 打算 mean to 打算 ought to 应该 plan to 计划 want to 想要
(九)两个不定式由and, or, except, but, than 连接时，第二个可省略to，尤其是两个不定式紧密相连时。例如：
I intend to call on him and discuss this question again. 我打算去拜访他并和他谈谈这个问题。
I’d like to lie down and go to sleep. 我想躺下睡觉。
You’re free to talk or laugh here. 在这里你可以随便说笑。
I’m anxious to go and ask him about it. 我急于去问问他这个事。
Do you want to have lunch now or wait till later? 你想现在吃午饭还是再等会儿?
We had nothing to do except (to) look at the posters outside the cinema. 我们无事可做，只有看看电影院外面的招贴。
He had nothing to do except talk nonsense. 除了胡说八道，他没别的事干。
I would rather die than be insulted. 我宁死也不受侮辱。