解析過(guò)程：
1.動(dòng)詞不定式做主語(yǔ)時(shí)可以放在句首,常表示某次具體的行為, 此時(shí)也可以用it做形式主語(yǔ)替代.例如：
To discuss the problem with an experienced teacheris necessary .=
It’s necessary to discuss the problem with an experienced teacher. 與一位有經(jīng)驗(yàn)的老師討論這個(gè)問(wèn)題是有必要的.
2.動(dòng)詞不定式做目的狀語(yǔ)時(shí)可以放在句首,也可以放在句尾 ,例如：
To tell you the truth, I don't like the way he talked. 說(shuō)實(shí)話,我不喜歡他講話的方式.
He worked day and night to get the money. 他夜以繼日地工作來(lái)賺錢.

She sold her hair to buy the watch chain. 她賣掉了自己的頭發(fā)來(lái)買那條表鏈.
規(guī)律方法：
動(dòng)詞不定式用法總結(jié)

動(dòng)詞不定式在句中可充當(dāng)主語(yǔ)、表語(yǔ)、賓語(yǔ)、賓補(bǔ)、定語(yǔ)和狀語(yǔ)等.
一、作主語(yǔ)
動(dòng)詞不定式作主語(yǔ)可位于句首.例如：
To learn a skill is very important for everyone in society.
也可使用it作形式主語(yǔ),而將其置于句末.例如：
It is necessary for young students to learn a foreign language.
動(dòng)詞不定式作主語(yǔ)的常用句型有：
1. It is＋adj./ n. (＋for sb.／sth.) ＋ to do sth.
用于此句型的形容詞有：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. nowB. man C. thatD. it
用于此句型的名詞有：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.
該句型中只能使用描述某人的品德、特征的形容詞,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.
該句型意為“做某事花費(fèi)某人多長(zhǎng)時(shí)間”.例如：
It took us half an hour to ride to the town by bike.
二、作表語(yǔ)
動(dòng)詞不定式作表語(yǔ)常用于以下結(jié)構(gòu)：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.
三、作賓語(yǔ)
常見(jiàn)的只能使用動(dòng)詞不定式作賓語(yǔ)的動(dòng)詞有：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等動(dòng)詞可用于“動(dòng)詞+ it +adj. / n+to do sth.”句型,其中使用it作形式賓語(yǔ),而將真正的賓語(yǔ)動(dòng)詞不定式置于句末.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.
四、作賓補(bǔ)
可后接動(dòng)詞不定式作賓補(bǔ)的動(dòng)詞有：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等動(dòng)詞后可接to be型不定式作賓補(bǔ).例如：
He declared himself to be a college student. 他自稱是名大學(xué)生.
The police proved him to be a thief. 警察局證實(shí)他是小偷.
hope, demand, suggest等動(dòng)詞不能后接動(dòng)詞不定式作賓補(bǔ).例如：
【誤】I hope my son to be back soon.
【正】I hope my son will be back soon.
【誤】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.
在主動(dòng)結(jié)構(gòu)中,下列動(dòng)詞后作賓補(bǔ)的動(dòng)詞不定式應(yīng)省略to：“五看”(see, watch, notice, observe, look at)“三使”(make, let, have)“兩聽(tīng)”(hear, listen to)“一感覺(jué)”(feel).例如：
Who made him work all night long?
但是,改為被動(dòng)結(jié)構(gòu)后,應(yīng)補(bǔ)出省略的to.例如：
He was seen to break the window.
五、作定語(yǔ)
動(dòng)詞不定式作定語(yǔ),應(yīng)位于所修飾詞語(yǔ)之后,即：作后置定語(yǔ).例如：
Have you got anything to eat? (to eat修飾anything,位于其后)
下列名詞后常接動(dòng)詞不定式作定語(yǔ)：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以及序數(shù)詞或形容詞最高級(jí)修飾的名詞后,也常接不定式作定語(yǔ).例如：
Who was the last one to leave the classroom last night?
六、作狀語(yǔ)
動(dòng)詞不定式作狀語(yǔ),可表示目的、原因、結(jié)果或條件.例如：
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.（結(jié)果）
To look at the picture, you would like it.（條件）
作目的狀語(yǔ),還可以使用in order to或so as to.例如：
The boy worked so hard in order to make up for the lost time.
結(jié)果狀語(yǔ)還可以使用enough to, too…to…, so…as to, such… as to等結(jié)構(gòu).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,以致沒(méi)有注意到我.如果還有其他疑問(wèn),可以去“狀元365”提問(wèn).