知识点：《虚拟语气的用法》收集：俞泳欠编辑：茉莉花仙子
本知识点包括：1、虚拟语气的各种用法 2、一种虚拟语气的用法：Be it.... 3、if引导的条件状语从句与虚拟语气的用法 4、求虚拟语气的用法，特别是should型虚拟。 5、虚拟语气的用法 。
《虚拟语气的用法》相关知识
but for要不是 本身就含有对于已发生的事情的一种假设,所以表示的是过去的某一个状态举个例子：But for his help ,i couldn't have made such big progress.要不是他的帮助,我不会取得如此大的进步!
给你一个公式（基本上是这样）：虚拟语气里,表示对现在虚拟的时候,主句谓语动词do变did,从句变would do；表示对过去已发生的虚拟是,主句变have done从句变would have done（注,有时would可用could、might、should等词语代替
知识拓展：
1：otherwise或asif以及butfor引导的虚拟语气的用法
知识要点归纳：
otherwise ： 用otherwise的虚拟条件句中,前一部分是陈述语气,后一部分是虚拟语气. 如：I ran all the way to school , otherwise I would have been late . 我一路跑到学校,...
2：otherwise,butfor,without虚拟语气里怎么用结构是什么?
知识要点归纳：
without, otherwise, but for, but引导的短语或句子有时相当于一个虚拟条件句.假设的情况并不以条件从句表示出来,而是暗含在上下文中,通过介词短语来表示.
使用到这些词组的句子中,虚拟语气的用法一般是：对现在的虚拟：would+do；对过去的虚拟：would(not) + have done.
比如：But for your reasonable advice,i wouldn't have fininshed my paper excellently.Without bring in him as the PM(project manager项目经理)of our company,the important program would have failed.
I should study more hard,otherwise i would have kepet with the most outstanding sutdent in my class in last exam. Your mother would go with you,but she has to take care of your elder brother
3：关于虚拟语气butfor含蓄条件句的用法Butfortheopeningpolicy,thesituationinChina_________whatitistoday.AwouldneverbeBwillneverhavebeenCwouldneverhavebeenDwillneverbe请问：是否为错综条件句的虚拟语
知识要点归纳：
是错综条件句的虚拟从句,要根据从句的时态,判断主句相对应的时态.因为是过去某件事的发生与否,对现在造成的影响,所以要用现在完成时.在给两个例句供参考：
But for my help,she would not have passed the exam.若不是我的帮助,她就不会通过考试.But for the atmosphere plants would die.如果没有大气,植物就会死亡
4：【虚拟语气中butfor,ifnot,butthat的意思及用法】
知识要点归纳：
亲自挨个查的朗文词典,敲上去的
but for
a) used when you are saying that something would have happened if something or someone else had not prevented it:(译成汉语：要不是）
But for these interruptions,the meeting would have finished earlier.
The score could have been higher but for some excellent goalkeeping by Simon.
I might never have got to university but for you.
b) except for something or someone:(译成汉语：除了）
All was silent but for the sound of the wind in the trees.
if not（如果不是/没有的话）
Your car should be ready by 12 o'clock,but if not I'll let you know.
If you don't leave now,I'll call the police.
Try these gloves on.If they're not the right size I'll take them back.
关于,but that,干脆多说几句,顺便讲一下but的用法
在英语中,but除了最常用的表示转折关系外还有以下几种常见用法：
一、but用法连词,表示让步关系,意思是“除非,要不是”,常与that一起构成but that,相当于"if...not".例如：I would have failed but that your helped me.要不是你们帮助我,我就会失败.
二、but用作连词,放在否定词或疑问词之后,作从属关系,表示否定意义,可相当于that not.例如：Never a month passed but she writes to her parents.她没有一个月不给她双亲写信.
三、but用于否定词加doubt,question,deny等到之后,没有实在意义,只相当于关系连词that.例如：There's no doubt but he is a thief.毫无疑问,他是一个贼.
四、but用作介词,表示“除了”意义,相当于besides,except.例如：No one knows him but she.除了她,没有人认识她.
五、but用作副词,相当于only,常译为“只不过,仅仅”.例如：He finished his homework but ten minutes ago.他十分钟前刚完成作业.
六、buy用作代词,表示从属关系,在否定句中常相当于who/that not.例如：There is no one but likes to help him.没有人不认识他.
5：butfor引导虚拟语气的句子紧接着butfor的部分是不是就是从句
知识要点归纳：
看这样的例子：
But for your help,I couldn't have finished it so successfully!
=Without your help,I couldn't have finished it so successfully!
=If it had not been for your help,I couldn't have finished it so successfully!
but for,without,or,otherwise等,引导的虚拟结构叫【含蓄条件句】,本质上不是从句,只是短语结构,但是能够表达虚拟条件,可以替换为虚拟条件句.替换方式见上面.
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