英语虚拟语气befor

不论主语为第几人称，if从句中的be动词均用were；主句中的助动词一定为过去时，且后面接动词原形

一、虚拟条件句：条件状语从句是非真实情况，在这种情况下要用虚拟语气。1．条件从句与现在事实不一致，其句型为：句型1：If ＋主语＋过去时，主语＋ should (could, would, 或might) ＋动词原形(1) If I were you, I would study hard.(2) If it rained, I would not be here now.2．条件从句与过去事实不一致，其句型为：句型2：If ＋主语＋had＋过去分词，主语＋ should（could, would, 或might）＋ have ＋过去分词(3) If the doctor had come last night, the boy would have been saved.(4) If I had not studied hard, I would have failed in the exam last term.3．条件从句与将来事实不一致，其句型为：句型3：If ＋主语＋过去时/ should ＋动词原形/ were to＋动词原形，主语＋ should (could, would, 或might) ＋动词原形(5) If it should rain tomorrow, we would stay at home.(6) If I were to go to the moon one day, I could see it with my own eyes.(7) If you missed the film tonight, you would feel sorry.注意问题：1．If条件句中绝对不可以出现would。2．根据句中的时间状语，有时可能出现“混合虚拟”的情况，即主句可能是现在的情况，条件句也许是发生在过去的情况，但都要遵守上述句型。如：(8) If you had studied hard before, you would be a college student now and you would graduate from a college in four years’ ti

before和ago都能表示在一段时间以前,但是二者的内涵和具体用法有明显的区别：1.before表示从过去或将来某一时间算起多少时间以前,由于与这一时间状语连用的行为先发生在过去或将来某一时刻之前,因此往往出现在过去完成时态或将来完成时态中.例如：① I called him up last night,but his mother told me he had left for Wuhan two days before.② She said that she had married him five years before.2.如果不具体表明多少时间以前,before意为"从前；以前",表示从现在开始追溯到过去一段时间,或者主句的谓语动词表示的是经常、反复发生的行为,以副词的形式置于句末,可以用在现在完成时态中.例如：①I have never been to Beijing before.②Have you ever seen a tiger before?③ I often go over the lessons that I have learned a few days before.注意：before前的时间状语如果是特定的,含义不一样.试比较：He said he had arrived here a day before.(一天前)He said he had arrived here the day before.(前一天)3.ago指的是从现在起到过去时间的某一点的一段时间.这个时间状语常与一般过去时态或过去进行时态连用.例如：① I graduated from the college twenty years ago.②I was asked what I was doing a month ago.4.表示在某一时间点或事件以前时,只能用before,不用ago.因为这种用法是将before当作介词或连词使用.ago不具有这种功能.例如：① He will come back before eleven o'clock.② Where did your brother study before he joined the army?③ It was hoped that this would be achieved before the year 2006.5.按语法规则,ago只能与一般过去时连用,但在since three years ago这类短语中能与现在完成时态连用.因为three years ago构成一个名词短语,共同作since的宾格,意为"自从三年前以来".例如：① He has been here since three years ago.② I have taught English in this school since twenty years ago.6.含before的从句一般不用将来时态,如果采用"will / would +V."时,一般被认为是情态或虚拟语气.例如：I would die before I would confess to it.7.如果before从句是一般过去时态,所暗示的是过去发生的事,主句则宜用过去完成时,语义偏重于过去某阶段的行为或状况时,还可与一般过去时态连用.例如：① I had never seen a Red Army man before I arrived in Xi'an.② They asked me things that happened before the boy was born.8.不少的语法书上说,before引导的从句习惯上不与否定的主句连用,要用until.实际上二者都可以,有时在意义上有些区别.试比较：He didn't arrive until I returned.直到我回来时他才到达.He did not arrive before I returned.在我回来之前他尚未到达.I didn't manage to do it until you had explained how.直到你解释了如何干,我才设法完成了那项工作.I didn't manage to do it before you had explained how.你解释了如何干之后,我才设法完成了那件工作.9.before在句中的含义比较灵活,虽然before引导的也是一个时间状语,但译成汉语时却不必死译为"在……以前".可以采取下面几种译法：(A)译为"……才"；(B)译为"……就"；(C)译为"尚未"、"还没有"、"来不及"、"不要"、"别"；(D)译为"在……的时候还没有"；(E)译为"如果"、"否则"、"免得"；(F)译为"到"、"等到"、"没等……就"；(G)译为"然后"等.例如：A.It will be five years before we meet again.要等五年我们才能再见面.We waited a long time before the train arrived.我们等了很长时间火车才到达.B.I hadn't waited long before he came.我没等多久他就来了.He had not gone a mile before he felt tired.还没走一英里路,他就觉得累了.C.His father died before he was born.他尚未出世他父亲就去世了.Catch him before he escapes.抓住他,别让他跑了.Take this computer before you change your mind.买了这台电脑,不要三心二意了.D.We arrived there before it began to rain.我们到达那里的时候还没下雨.E.Go before I call the police.走吧,否则我叫警察了.I had better go now before it is too late.我最好现在就走,免得太迟.Pigs will fly before he'll be a scientist.如果他当科学家,猪都会飞了.F.The crops will die before the rain fall.等到天下雨,庄稼早枯死了.Before I could sit down,he offered me a cup of tea.没等我坐下,他就给我端上一杯茶.G.I shaved before I went to the party.我刮了脸,然后去参加宴会.

ago和before的区别是：ago立足于现在，表示从现在起，若干时间之前；before立足过去，表示从过去某一时刻起，若干时间以前。ago 通常与一般过去时连用，不能与完成时连用；before通常与过去完成时连用。例如： I met him three years ago .（距今）三年前，我遇到他。 I had met him three years before .（距当时）三年前我见到他一次。