英语什么情况下用不定式?举例..重酬!

不定式在句子中可做主语、宾语、表语、定语、状语和宾语补语.
　　1）不定式做主语一般表示具体的某次动作.
　　【例如】
　　To complete the 30 storied building in one year was quite a difficult task.
　　To do that implies taking responsibility.
　　当主语较长,谓语较短时,常用it做形式主语,而将不定式放到谓语的后面.
　　【例如】
　　It is important for modern young people to master at least two foreign languages.
　　It做形式主语,不定式放在谓语动词之后常用于下列结构中：
　　a） It is＋形容词（easy,important,difficult,foolish,inconvenient,unnecessary,right,wrong...）＋（ for/of sb.）＋不定式
　　【例如】
　　It is essential to reserve a table in advance of Christmas Eve.
　　It is hard to put my hopessintoswords.
　　It is not easy to catch fish with your hands only.
　　It is important for us young people to learn English and master it.
　　b） It is＋名词（a pleasure,a pity,a pleasant thing,one's duty,an honor,a shame,a crime,no easy job...）＋不定式
　　It is a sheer waste of time to read that kind of trash.
　　It is a pity to have to go without her.
　　It is a glorious death to die for the people.
　　c） It takes （sb.） some time （hours,months,days,a lot of time,patience...）＋不定式
　　It takes me three hours to learn English each day.
　　It took them half the night to get home in the snow.
　　2）不定式作表语不定式作表语常表示将来的动作,主语常常是表示意向、打算、计划的词,如wish,idea,task,purpose,duty,job等.
　　【例如】
　　The most important thing for one's health is to have plenty of exercise.
　　My chief purpose has been to point out the difficulties of the matter.
　　The purpose of the organization is to greet all new comers to the city and to provide them with any necessary information.
　　What I wanted was to get the work done as quickly as possible.
　　3）不定式作宾语不定式常在下列动词后面作宾语：
　　afford,agree,apply,arrange,ask,attempt,beg,begin,care,choose,claim,consent,demand,decide,desire,determine,expect,fail,hope,hesitate,hate,intend,learn,like,manage,mean,neglect,offer,plan,prepare pretend,promise,refuse,resolve,seek,tend,threaten,want等.
　　【例如】
　　I like to go out for walks in the warm sunshine in spring.
　　My mother hates to move from place to place,for she feels tired.I decide to work hard and get doctor's degree.
　　The room is designed to be my study,but now it has to be used as a bedroom for the children.
　　She failed to finish the assignment in time,and she was worried about it.

to+do就是不定式，除了加doing的keep，finish，enjoy，mind，practice和加do的feel（一感） ，listen，hear（二听），let，make，have（三让），see，watch，find，notice（四看），help（半帮助）以外，所有的动词都加to do。

表目的，比如I go to Beijing to see the Olmpicgames

动词不定式概述
一 不定式的体
1. 不定式的一般式(to do)表示不定式的.动作将要发生或与谓语动作同时发生.
2 不定式的进行体(to be doing)表示不定式的动作正在发生.
不定式的完成体(to have done)表示不定式的动作在谓语动作之前发生.
二 不定式的逻辑主语
1 不定式的逻辑主语常不出现在句中.
2...