不定式的构成非常简单：to do sth (do 代表动词原形)，其否定形式是not to do sth，被动形式：to be done 有时也可以不带to。一般表示要干还没干的事（干过的事一般用-ing形式）。动词不定式没有人称和数的变化， 具有名词、形容词和副词等的功能，可以担当除谓语外的任何句子成分，即：主语、宾语、表语、定语、状语、补足语、同位语。
不定式作主语：一般位于句首，谓语用单数
To go to college is our ideal. 上大学是我们的理想。
To learn foreign languages is difficult. 学外语很难。
为了平衡句子，通常用it做形式主语替不定式，即：
It’s+名词/形容词+for/of sb.+to do sth. (注：此句式中不定式逻辑上的主语可由for或of引出，当表语的形容词为修饰逻辑主语sb. 的kind，nice，good，polite，clever，foolish，right，wrong等时，逻辑主语由of引出)
It takes (sb.) some time (hours, months, days, a lot of time, patience...)＋to do sth
则上面两句话可变为：
It’s our ideal to go to college.
It’s difficult (for us) to learn foreign languages.
又如：It’s kind of you to say so. 你那样说真好。
不定式作宾语：
I like to watch TV. 我喜欢看电视。
He wants to swim. 他想要游泳。
不定式作表语：常用在系动词be, seem, appear, get, remain等后作表语，如：
The meaning of the word is to go quickly. 这个词的意思是快点走。
To the doctor, the most important thing is to save the lives. 对于医生来说，最重要的是治病救人。
This suit doesn’t seem to fit her. 这套衣服似乎不合她身。
不定式作定语：
不定式作定语通常放在其修饰的名词或代词之后，和被修饰的名词或代词之间是主谓关系、动宾关系、同位关系和修饰关系。
1 ．主谓关系 Yang Liwei is the first Chinese to go into the space. 杨利伟是第一个登上太空的中国人。
The future to greet us will be bright. 我们的未来会十分美好的。 The next train to arrive was from New York. 下一列到站的火车是从纽约开来的。
2 ．动宾关系 He has something important to do. 他有重要事情要做。
On Sundays, he always has a lot of letters to write. 星期天，他总是有许多信要写 The teacher left us a lot of problems to solve. 老师给我们留了许多道题要解答。 He can find no one to make friends with. 他找不到可交朋友的人。 注意：由于动词不定式与其修饰的词之间往往有动宾关系，因此，如果该不定式是不及物动词，其后应有必要的介词。 She has a child to take care of. 她有一个孩子要照看。 There is nothing to worry about. 没什么可担心的。 He has no friend to depend on. 他没有可依靠的朋友。 I've got a lot of things to see to this morning. 今天上午我有许多事情要处理。
3 ．同位关系 He broke his promise to write to his parents regularly. 他没有遵守诺言定期给他父母亲写信。 We students should have the courage to face any difficulty. 我们学生应该有勇气面对任何困难。 I have no wish to quarrel with you. 我不想和你吵架。
4 ．修饰关系 Now it is time to begin our class. 现在是上课的时间了。 I need somewhere to take a nap. 我需要一个地方打个盹。 Is that the way to open the can 那就是打开罐头的方法吗？
不定式作状语：
1 ．表示目的 The foreigners came to China to see the Olympics. 这些老外来中国看奥运会。（目的状语）
I'm saving up to buy a computer. 我在存钱买电脑。 To save the child, he laid down his life. 为了救那个孩子，他献出了生命。 注意：有时为了强调或突出这种目的，也可以用in order (not) to do,和so as (not) to do结构（so as to do不可以置于句首）。 He shouted and waved in order to be noticed. 为了吸引注意，他又嚷嚷又挥手。 I'll write down his telephone number so as not to forget it. 我要把他的电话号码记下来，以防忘记。 2 ．表示结果 The box is too heavy for me to lift. 这个箱子太沉了，我提不动。
He is old enough to go to school. 他到上学的年龄了。
He got to the station only to find the train had gone. 他赶到车站，发现火车已经开走了。 What have I said to make you so angry 我说了什么话使你气成这样？ After the meeting, they parted, never to see each other again. 散会后，他们分手了，从此再也没有见面。 必背：不定式表示结果常见于下列句型。 1） so ... as to do Would you be so kind as to lend me your bicycle 把你的自行车借给我好吗？ 2） such ... as to do We are not such fools as to believe him. 我们还没蠢到竟会相信他。 3） enough… to do He didn't run fast enough to catch the train.他跑得不够快, 没赶上火车。 4） only to do He lifted a rock only to drop it on his own feet. 他搬起石头砸自己的脚。 5） too ... to do His eyesight is too poor to read such small letters. 她的视力太差了，不能看这么小的字。 注意：too...to...结构中的形容词如果是eager, pleased, happy, ready等，动词不定式不表示结果，也没有否定的意思。 The boy was too eager to get a geography book. 那个男孩非常急切地想要得到一本地理书。 He is too anxious to know the examination results. 他很急切地想知道考试结果。 3 ．表示原因 She is glad to meet us. 见到我们她很高兴。
I was a fool not to listen to you at that time.我当时不听你的话，真是傻瓜。 She wept to find him in such a difficult situation. 看到他处于这么困难的情形，她哭了。 4 ．表示条件 A man would be blind not to see that. 一个人如果看不到这点，那他就是瞎子。 How can you catch the train to start so late 这么晚才动身，你怎么能赶上火车？ 不定式作补语：
如：warn, ask, allow, permit, order,
advise, expect, wish, force, beg, +sb/sth+(not) to do sth
invite, encourage, persuade, tell
不定式作同位语
His task, to clean the room, is very easy. 他的任务，即打扫房间，是很简单的。
不定式省to的情况： 一感：feel?? 二听：hear, listen to? 三让：let, have, make? 四观看：observe, see, watch., look at
使役动词 let, have, make 等后接不定式。如：Let him go! 让他走!
感官动词 see, watch, look at, hear, listen to, smell, feel, find 等后作宾补，省to。如：I saw him dance. 我看见他跳舞。
would rather…than(宁愿……也不)，had better后。如：
You had better stay at home.你最好呆在家里。
I would rather stay at home than go there. 我宁愿呆在家也不去那。
Why... / why not...后。如：Why not have a good rest on Sunday? 为什么星期天不好好休息一下呢?
rather than, other than之后，如：Rather than ride on a crowded bus, he always prefers to ride a bicycle. 与其挤公共汽车，他总是更喜欢骑自行车。
注意：以上这些情况在被动句中可千万不可省to 哟！ 如：
The boss made them work the whole night. 变成被动句： They were made to work the whole night.
不定式的特殊用法：
It作形式主语：动词不定式可以做主语，但如果动词不定式太长，显得头重脚轻的，那么我们就可用形式主语it代替，而把真正主语(即不定式)放于句尾。如:
It is not difficult for me to study English well. 对我来说学好英语 。
It作形式宾语：不定式还可以充当句子的宾语，但有些动词，如find，think，believe 等，在语法上不能接受不定式作宾语，只有用it作形式宾语，从而把动词不定式置于句尾。这样的不定式可继续充当其宾语的作用。如:
I found it difficult to fall asleep.我发现很难入睡。
动词不定式还可用在how，when，where，what，which 等疑问代词或副词之后，与其共同作宾语。如：
I don't know how to use a computer.我不知道怎样使用电脑。
Can you tell me where to go? 你能告诉我往哪走吗？
Mr Green didn’t know what to do. 格林先生不知道该怎么办。
有些动词后面既可+to do sth., 也可+doing，但含义不同。to do sth. 往往表示要干的事，而doing sth. 表示干过的事。如：
go on doing sth.继续干同一件事 go on to do sth.接着干另外一件事
stop doing sth.停止干某事 stop to do sth.停下来去干某事
forget doing sth.忘记已干了某事 forget to do sth.忘记要干某事
remember doing sth.记得已干了某事 remember to do sth. 记住要做某事
try doing sth.尝试干某事 try to do sth.尽力/设法做某事
mean doing sth.意味着做某事 mean to do sth.打算干某事
can’t help doing sth.情不自禁干某事 can’t help (to) do sth.不能帮助 某事
regret doing sth.后悔干了某事 regret to do sth.因要做某事而遗憾
温馨提示：答案为网友推荐，仅供参考