until,before和after的用法差异这三者在用法上的差异,讲全一点,要有例句,并且请告诉我在句中使用时,这个词的前面,后面的动词要用什么时态.答得好的我会给出5分.

　　介词before 与 after 的用法
　　before”与“after”?
　　2008-01-16 18:16:28
　　用汉语表达时间的先后时,人们通常用“以前”和“以后”,词的变化不大,而且重复率非常高.受此影响,不少学生在英语里也千篇一律地用“before”和“after”来表达这两层意思.其实在英语词汇里可以表达这两层意思的词有很多个,其中有几个使用频率特别高,但它们在用法上有差异,极易混用.掌握这些词的用法,正确区分它们的差异,可以帮助我们写作时避免用词重复,结构单调,使文章生动.由于这些词属于高频词,把握好这些词,对准确理解和翻译也不无帮助.下面就对相关词进行归纳和比较.
　　一、表达“在……之前”的词
　　1. before prep. , adv. conj.
　　(a)To my knowledge it has never been done before.
　　据我所知,以前从未有人这样做过.
　　(b)My father died before I was born.
　　我父亲在我出生之前就去世了.
　　(c)Before being supplied to houses, the water must be treated to make it suitable for drinking.
　　水在供给各用户之前应加以处理使之适于饮用.
　　2. until prep. , conj.
　　(a)Until the twentieth century man used water, wood and fossil fuels as sources of power.
　　在20世纪以前,人们使用水、木柴和矿物燃料来作能源.
　　(b)Until penicillin was developed, man could do little with the disease.
　　在研制出青霉素之前,人类对这种病毒几乎无能为力.
　　(c)Color was a mystery until the time of Newton.
　　在牛顿时代以前,颜色是个神秘的东西.
　　〔注〕：before与until当连词用时,都表示主句的情景发生在从句情景之前,它们主要有两个区别：
　　第一,带有until从句的肯定式主句中的谓语动词必须是持续性的（如be, live, like, work等）,而带before从句的主句中的谓语动词则不必是持续性的.比较：
　　(a)We always have to wait until the last passenger has left.
　　我们总是得等到最后一个乘客离去.
　　(b)I shouted before the boss arrived.
　　在老板到来之前,我大喊大叫.
　　第二,until引出的分句的内容是真实的,而before引出的分句的内容可能是真实的,也可能是非真实的.比较：
　　(a)Let's wait until the rain stops.
　　让我们等到雨停吧.
　　(b)I began to cry before he could speak.
　　他尚未开口,我就开始哭了.
　　(c)Pigs will fly before he'll become a mathematician.
　　他要是能当数学家,猪都会飞了（在他成为数学家之前猪都会飞了.意即根本不可能）.
　　3. Previous a. ,adv.
　　(a)With the help of his coach, he has recovered to the previous level.
　　在教练的帮助下,他已经恢复到以前的水平.
　　〔注〕：previous当副词用常与to连用
　　(b)You are expected to make full investigations previous to reaching a conclusion.
　　希望你们下结论之前先作先分调查.
　　(c)Previously, the local people used wood and coal to provide power and heating.
　　以前,当地人用木柴和煤来提供动力和燃料.
　　4. prior (to) a. ,adv.
　　(a)It happened prior to the warning of the government.
　　这发生在*发出警告以前.
　　(b)The contract will be signed prior to the ceremony.
　　合同将在仪式之前签署.
　　(c)It is believed that the skill was developed prior to Ming Dynasty.
　　人们相信这项技术在明朝以前就已经发明了.
　　〔注〕：prior to和before虽然可以通用,但有人认为prior to最好避免用在句首.
　　二、表达“在……之后”的词
　　1. after prep. , adv. conj.
　　(a)After testing, the engine was put into immediate service.
　　经过试验以后,发动机立即交付使用.
　　(b)It happened twenty years after.
　　这事发生在20年之后.
　　(c)After one cycle is completed, another cycle begins.
　　一个循环完成后,另一个循环又开始.
　　2.in prep.
　　(a)In about five years, what I have said today will become true.
　　大约5年以后,我今天所说的就会变成现实.
　　(b)In the four years that followed, I studied in the Department of Accounting.
　　在以后的4年中,我在会计系学习.
　　(C)It's two o’ clock. I'll come in an hour.
　　现在是两点,我一小时后到.
　　〔注〕：在表示以现在为起点的“在……之后”这个含义时,介词in之后应接时间段(a period of time),介词after之后则应该接时间点(a point of time).比较：
　　(a)I've ordered a taxi. It will arrive here in 10 minutes.
　　我已经叫了出租车,10分钟以后就会到.
　　(b)He will graduate in four years.
　　他4年以后毕业.
　　(c)He will be back after three o'clock.
　　他3点以后到.
　　但是,当以某个过去的时间为起点时,after之后既可以接时间点,也可以接时间段.比如：
　　(a)Just after six the meeting was over.
　　刚过6点,会议就结束了.
　　(b)After half month of war Japan was badly defeated.
　　经过半个月的战争,日本被打得惨败.
　　(c)After seven days of this I went to a party.
　　这样过了7天以后,我去参加一个聚会.
　　3. later
　　(a)Fuller report will be published at a later date.
　　更详细的报告会在以后出版.
　　(b)I don't know what would happen later on.
　　至于以后怎么样,我就不知道了.
　　(c)He handed in his resignation, and later (on)regretted his hasty action.
　　他交了辞职书以后,又为自己的草率感到后悔.
　　〔注〕：later还可译为“过一会儿”,“过了一会儿”,“后来”等.
　　4.afterward(s) adv.
　　(a)She had her supper and went bed soon afterwards.
　　她吃过晚饭之后不久就去睡觉了.
　　(b)I didn't see him afterwards.
　　以后我再也没有见到他了.
　　(c)I was a bit rude to her at the meeting, and afterwards I apologized.
　　在会上我对她是有点粗暴,但是（散会）以后我向她道了歉.