概論：不受主語的人稱和數的制約，也不體現時的特征，不能獨立充當謂語，只能與一定的助動詞結合構成進行體、完成體或被動語態。
形式：不定式、分詞、動名詞。
I. 不定式
一、 主語（體現名詞特征）
To know the main facts is not enough.
It is a great pleasure to be here.
It made me happy to find Marry there.
Is it a good idea to go swimming today?
What fun it is to read Mark Twain’s book!
It’s important for us to master English.
It’s kind of you to say so.
二、 表語（體現形容詞特征）
To see is to believe.
To be kind to the enemy is to be cruel to the people.
My idea is to climb the mountain from the north.
Cf. I am to leave tomorrow.
You are to answer for it.
三、 賓語（體現名詞特征）
1.agree, ask, choose, decide, demand, determine, expect, hope, learn, like, love, long, manage, offer, pretend, promise, refuse, want, wish等后面可以接賓語不定式。
He agreed to smoke no more.
What did he say he wanted to do?
2.decide, find out, forget, know, learn, remember, see, wonder后可以接不定式特殊結構。
what to do.
where to go.
whom to ask.
I don’t know who to do it.
when to stop.
how to get there.
Which to choose.
從例句得出不定式特殊結構的規律：沒有進行時態、沒有被動語態、沒有why.
四、 賓補（體現形容詞特征）此時賓語是不定式的邏輯主語。
1. 感覺動詞see, hear, feel, watch, notice, observe, listen to, look at。
Did you see him go out?
2. 表示“致使”等意思的動詞have, let, make, advise, cause, allow, ask, beg, encourage, expect, forbid, force, get, invite, permit, persuade, teach, tell, want, warn等。注：有下劃線的動詞接省略to的不定式賓補。
I made him leave without any hesitation.
I can’t get her to talk.
3.心理狀態的動詞consider, declare, find, prove, believe, discover, feel, judge, suppose等。
We found him to be dishonest.(to be可以同時省略)
4.短語動詞long for, would like, prepare for, wait for, call on等。
The Party has called on us to learn from Lei Feng.
五、 定語（體現形容詞特征）
It’s time to go to bed.
Have you anything to say for your self?
I have no wish to quarrel with you.
I’ve brought this chair to sit in.
You’ve given me much to think about.
He’s always the first to come and the last to leave.
六、 狀語（體現副詞特征）
1.目的狀語(句首或句末)
He was running to catch the bus.
She opened the door for the children to come in.
I took my shoes to be mended.
He went early in order/ so as not to miss the train.
(In order) To draw maps properly, you need a special pencil.
2.結果狀語（句末）
She went abroad, never to return.
Would you be so kind as to lend me some money?
He is such as fool as to think he is right there.
The ice is thick enough to walk on.
That’s much too heavy for you to lift.
She woke up early to find it was raining.
I got to the station only to be told the train had gone.
3.原因狀語（句末）
He laughed to see such fun.
She seemed surprised to meet us.
I pretended to be happy to know him.(原因)
I pretended to be happy to stop my mother from worrying.(目的)
原因不定式與目的不定式的區別：（1）如果不定式前可以加上in order或，就是目的狀語；（2）如果表示目的不定式結構表示的動作后于謂語動詞的動作；如果表示原因，不定式結構表示的動作先于謂語動詞的動作。
*關于不定式與形容詞搭配
A．句子的主語是不定式結構的邏輯主語。
（1） 表示喜怒哀樂的形容詞angry, glad, happy, proud, sorry, thankful, astonished, delighted, disappointed, excited, interested, moved, pleased, satisfied, surprised, worried等；
He was delighted to see his old friend again.
（2） 表示運氣的形容詞lucky, fortunate等；
We are lucky to make such a good friend.
（3） 表示心理狀態、個人態度的形容詞afraid, anxious, determined, eager, free, ready, willing, prepared, able, disable等；
She was afraid to go out at night.
(4) 表示可能性、確定性的形容詞likely, certain, sure;
*He is likely to see Marry.= He will probably see Marry.
=It is likely that he will see Marry.
*She’s certain to do well in the exam.= She will certainly do well in the exam.
= It is certain that she will do well in the exam.
*James was sure to need help.= James will surely need help.
=James will need help, that’s sure.
(5)表示性格特征、行為表現的形容詞brave, clever, cruel, foolish, good, honest, kind, lazy, modest, nice, polite, rude, selfish, silly, stupid, wise等；
John was careless to break the cup.
=It was careless of John to break the cup.
轉換成由how引導的感嘆句時，it is/was常省略，如上句：
How careless of John to break the cup!
B．句子的主語是不定式結構的邏輯賓語。
（1）能轉換的
Mary is easy to teach.
=To teach Mary is easy.
=It is easy to teach Mary.
Mary is easy for you to teach.
=It is easy for you to teach Mary.
=For you to teach Mary is easy.
Mary is an easy pupil for you to teach.
=Mary is a pupil easy for you to teach.
這類句型中的形容詞通常為cheap, dangerous, difficult, easy, expensive, impossible, funny, interesting, pleasant, hard, important, improper, nice, splendid, strange, useful等。
如果不定式為不及物動詞，則要加上介詞或副詞，如：
Mary is pleasant to talk to.
Foreign countries are often exciting to go to.
（2）不能轉換的
The coffee is bitter to taste.
The grass is very smooth to walk on.
This map is clear to read.
These cookies are tasty to eat.
注意：不定式有時用被動語態與主動語態沒有區別，如：
This is too important a matter to leave/ to be left until tomorrow.
These books are not strong enough to use/ to be used as a platform.
There is so much work to do/ to be done.