To learn a skill is very important for everyone in society.
It is necessary for young students to learn a foreign language.
1. It is＋adj./ n. (＋for sb.／sth.) ＋ to do sth.
用于此句型的形容词有：easy, hard, difficult, possible, important, impossible, necessary, good, bad, exciting, interesting, surprising等。例如：
It is interesting to play this game.
It is necessary for you to change your job.
It was impossible for them to complete the task in such a short time.
考例1：Is ____ possible to fly to the moon in a spaceship? (88 MET)
A. now B. man C. that D. it
用于此句型的名词有：pity, shame, pleasure, one’s duty, one’s job, fun, joy, good manners, bad manners等。例如：
What a pity it is for you to have missed such a wonderful film.
It is good manners for the young to give their seats to the old.
2. It is＋adj.＋of sb.＋to do sth.
该句型中只能使用描述某人的品德、特征的形容词，如： kind, nice, wise, silly, polite, impolite, friendly, foolish, clever等。例如：
How rude it was of the boy to jump the queue!
It is friendly of the family to try to make me feel at home in their house.
How silly it was of you to give up such a good chance！
3. It takes sb.＋some time＋to do sth.
It took us half an hour to ride to the town by bike.
动词不定式作表语常用于以下结构：My wish/ job/ aim/ goal is…及The next step/ measure is …等。例如：
Your job is to type the papers in the office.
The next measure is to stop the river from being polluted.
常见的只能使用动词不定式作宾语的动词有：agree, choose, decide, hope, fail, wish, refuse, expect, manage, plan, intend, pretend, promise, offer, afford, demand, arrange等。 例如：They decided to build a highway between these two cities.
She offered to help me when I was in trouble.
believe, think, consider, feel, make等动词可用于“动词+ it +adj. / n+to do sth.”句型，其中使用it作形式宾语，而将真正的宾语动词不定式置于句末。例如：
I think it necessary for us to have a good rest after the long work.
She felt it her duty to help the old woman.
可后接动词不定式作宾补的动词有：advise, allow, ask, hear, order, see, tell, want, wish, watch等。例如：
The doctor advised her no to eat too much sugar.
I wish you to go to the meeting with me.
believe, consider, count, declare, deny, feel, find, guess, imagine, judge, know, prove, realize, suppose, think等动词后可接to be型不定式作宾补。例如：
He declared himself to be a college student. 他自称是名大学生。
The police proved him to be a thief. 警察局证实他是小偷。
hope, demand, suggest等动词不能后接动词不定式作宾补。例如：
【误】I hope my son to be back soon.
【正】I hope my son will be back soon.
【误】She suggests us to have a discussion about it.
【正】She advises us to have a discussion about it.
【正】She suggests that we (should) have a discussion about it.
在主动结构中，下列动词后作宾补的动词不定式应省略to：“五看”(see, watch, notice, observe, look at)“三使”(make, let, have)“两听”(hear, listen to)“一感觉”(feel)。例如：
Who made him work all night long?
He was seen to break the window.
Have you got anything to eat? (to eat修饰anything，位于其后)
下列名词后常接动词不定式作定语：ability, attempt, chance, courage, decision, effort, failure, promise, way, wish等。例如：
But she gave up the chance to go abroad.
由only, first, last, next以及序数词或形容词最高级修饰的名词后，也常接不定式作定语。例如：
Who was the last one to leave the classroom last night?
We went there to see our grandparents.（目的）
I am very sorry to hear that.（原因）
She hurried home only to find her father dead.（结果）
To look at the picture, you would like it.（条件）
作目的状语，还可以使用in order to或so as to。例如：
The boy worked so hard in order to make up for the lost time.
结果状语还可以使用enough to, too…to…, so…as to, such… as to等结构。例如：
He got up too late to miss the early bus.
She was in such a hurry as not to notice me. 她如此匆忙，以致没有注意到我。