1、不定式是一种非限定动词,通常由"to+动词原形"构成,如to be,to do,to combine,to expand,to get up等。这里的to是不定式的符号,没有词义,有时还可以省略。
2、动词不定式没有人称和数的变化。
3、不定式在句中不能单独用作谓语,但还保留着动词的若干特征:可以带状语,及物动词可以带宾语,系动词可以带表语。
4、不定式可以连同它的附属成分一起组成不定式短语。
不定式的用法：不定式(短语)在句中起相当于名词、形容词或副词的作用,可作主语、表语、宾语、定语、状语和宾语补语。
1、具有名词的性质
①用作主语。
例句：
（1）To live is to function.That is all there is in living.(Holmes)
活着就要发挥作用,这就是生活的全部真谛。
（2）To live of money is the root of all evil.(R.L.Stevenson)
迷恋金钱是一切罪恶的根源。
（3）To be ignorant of one's ignorance is the malady of the ignorant.(A.B.Alcott)不知道
自己的无知,是无知者的可悲之处。
ps：不定式、特别是不定式短语作主语时,为了保持句子平衡,往往把主语放在句子末尾,而在它原来的位置上加上引导词it作形式上的主语。
例句：
(1)To make you understand him completely is impossible.
→It is impossible to make you understand him completely.
让你们完全了解他是不可能的。
(2)To run machines needs power.
→It needs power to run machines.
开动机器需要动力。
(3)To remember to switch off the electricity is important.
→It's important to remember to switch off the electricity.
重要的是要记住关掉电源。
【注】"It is+形容词+of sb.to do"句型一般可转换成一个不定式作状语的句子,语义不变;而"It is+形容词+for sb.to do”句型则不可以。
②用作表语。
例句：
（1）Her wish was to become a teacher.
她的愿望是当个教师。
（2）The most dif?cult thing in life is to know yourself.(Thales)
人生最难是自知。
（3）To know everything is to know nothing.
样样都懂,样样稀松。
ps：1、带wh-疑问词的不定式短语可作表语。
例句：
Our difficulty is where to find a guide.
我们的困难是到哪里去找向导。
2、用介词for引出不定式的逻辑主语的不定式复合结构,可作表语;这时,句中的主语是不定式的逻辑宾语。
例句：This form is for you to fill in.
这张表格由你来填写。
③用作宾语。
例句：
（1）The owner wanted to make a large purchase of gift before Christmas.
老板想在圣诞节前做一大笔礼品生意。
（2）I try to keep that in mind.
我努力记住这一点。
（3）He had decided to return after receiving the first letter from Tess.
收到苔丝的第一封信后,他便决定起程回来。
ps：作宾语的不定式后面如果有补语,可以用it作形式宾语,把真正宾语置于补语之后,以免宾语太长使句子显得不协调。在这种用法中,常见的动词有think, feel, make, find, consider, count, deem,judge,believe,take等。
例句：
（1）She thought it unnecessary to quarrel with him over trifies.
她认为没必要为小事跟他争吵。
（2）I did not think it convenient to write about the matter.
我觉得写信谈这件事不合适。
（3）He found it utterly impossible to leave the spot.(T.Hardy)
他发现要离开这里是完全不可能的。
④用作宾语补语。
例句：
（1）I wanted something to happen to me,but nothing happened.(L.Hughes)
我希望在我身上会发生些什么变化,可是什么都没发生。
（2）I know nothing about him and I beg you to tell me nothing.
他的情况我并不知道,我也请求您别告诉我。
ps：在一些使役动词如make,let,have等和表示感觉的动词如see, watch, perceive, observe,notice,hear,feel,look at,listen to等,作宾语补语的不定式须省略to。
例句：
（1）She let her hand fall from his arm and said no more.
她放开他的胳膊,没再说什么。
（2）Electricity makes machines run.
电使机器转动。
（3）Then she turned quickly away,and Reinhard heard her go sobbing up the
stairs.
她很快转过身,莱因哈特听到她抽泣着走上台阶。
⑤用作主语补语。
例句：
（1）This test is thought to have practical value.
大家认为这个试验有实际价值。
（2）He was seen to enter the laboratory.
有人看见他走进了实验室。
⑥用作同位语。
例句：
Their aspiration,to become free and independent,is deserving of every kind
of help from us.
他们想自由和独立的愿望值得我们给予各方面的援助。
2、具有形容词的性质
①用作定语。
例句：
（1）Can you think up a good way to deal with the ever increasing prices?
你们能否想出一个对付日益上涨的物价的好办法?
（2）A letter from Clare gave her an excuse to leave.
克莱尔的一封来信给了她一个离开的借口。
（3）At present I have no lawful right to act for her.
目前,我还没有保护她的法律权利。
ps：如果不定式和被修饰的具体名词之间有逻辑上的动宾关系,则不定式多半表示未来的动作,翻译时常须在不定式前加"要"字。
例句：
（1）We have three machines to repair today.
我们今天有三台机器要修理。
（2）Do you like something to drink?
你要喝点什么吗?
3、具有副词的性质
①用作状语。
目的状语：
例句：
（1）We use electrical energy to do many things.
我们用电能做许多事情。
（2）He will be in Atlanta Friday to speak with Captain Butler and to escort you
home.
星期五他要到亚特兰大,同巴特勒船长交涉,顺便带你回家。
（3）John hurried in order not to be late for the party.
约翰急急忙忙的,为了准时参加晚会。
结果状语：
例句：
①After the war they parted company,never to see each other again.
战后他们分了手,从此再也没见过面。
②She went abroad never to return.
她到国外去了,从此没有回来过。
原因状语：
例句：
（1）He laughed to see such fun.
他看见这么可笑的事笑了。
（2）I trembled to think of it.
一想到这件事,我就不寒而栗。
（3）He jumped with joy to learn his being admitted to the university.
他知道被录取进大学高兴得跳起来。
条件状语：
例句：
（1）To hear him talk,you would think he owned the whole world.
如果你听他讲话,你会以为整个世界都是他的。
（2）A man would be blind not to see that.
一个人如果看不到这一点就是瞎子。
（3）They could have done better to have had some tools.
他们如果有些工具就会做得更好些。
方式状语：
例句：
（1）He cleared his throat as though to speak.
他清了清喉咙,好像要说话的样子。
（2）As if to justify his view,he cited some groundless rumors.
好像要证明他的观点正确似的,他引述了一些毫无根据的传闻。
比较状语：
例句：
（1）I know better than to believe such a man.
我不至于相信这样一个人。
（2）The American president had no better choice than to resign.
这个美国总统除辞职外,别无更好的选择。
让步状语：
例句：
（1）To do his best,he could not ful?l his task in time.
即使他竭尽全力,仍然不能按时完成任务。
（2）You couldn't do that to save your life.
你即使为了救自己的命也不能那样做。
4、用作插入语。
例句：
（1）To begin with,we must consider the problem in an all-round way.
首先,我们必须全面地考虑这个问题。
（2）Among the writers of his age,he was,so to speak,a giant among dwarfs.
在同时代的作家中,他可以说是鹤立鸡群。
（3）To tell you the truth,they didn't quite like the idea.
实话告诉你,他们并不很喜欢这种想法。