动词不定式的问题什么时候句子开头用不定式To.比如to find more about unniversity courses,write to this address.还有：bitten twice,the postman refused to deliver our letters unless we chained our dog.这句为什么不用 to be bitten 怎么区分 动词不定式表目的,将来 还有什么呢

to表目的时意为为了....，如果第二句用 to be bitten就是说为了被咬两次邮递员拒绝交付我们的信件,除非我们束缚我们的狗。这没可能吧。而且表目的的to只能放句首，表将来放句中。

解析过程：
1.动词不定式做主语时可以放在句首,常表示某次具体的行为, 此时也可以用it做形式主语替代.例如：
To discuss the problem with an experienced teacher is necessary .=
It’s necessary to discuss the problem with an experienced teacher. 与一位有经验的老师讨论这个问题是有必要的.
2.动词不定式做目的状语时可以放在句首,也可以放在句尾 ,例如：
To tell you the truth, I don't like the way he talked. 说实话,我不喜欢他讲话的方式.
He worked day and night to get the money. 他夜以继日地工作来赚钱.

She sold her hair to buy the watch chain. 她卖掉了自己的头发来买那条表链.
规律方法：
动词不定式用法总结

动词不定式在句中可充当主语、表语、宾语、宾补、定语和状语等.
一、作主语
动词不定式作主语可位于句首.例如：
To learn a skill is very important for everyone in society.
也可使用it作形式主语,而将其置于句末.例如：
It is necessary for young students to learn a foreign language.
动词不定式作主语的常用句型有：
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.
该句型意为“做某事花费某人多长时间”.例如：
It took us half an hour to ride to the town by bike.
二、作表语
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.
三、作宾语
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.
四、作宾补
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?
但是,改为被动结构后,应补出省略的to.例如：
He was seen to break the window.
五、作定语
动词不定式作定语,应位于所修饰词语之后,即：作后置定语.例如：
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*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?
六、作状语
动词不定式作状语,可表示目的、原因、结果或条件.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.（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.如果还有其他疑问,可以去“状元365”提问.