虚拟语气
（一）三种基本情况的谓语动词形式
所述情况
 从句
 主句
 
与现在事实相反
 if + 主语 + 动词过去式
（be动词用were）
 主语 + would/could/might/ should + 动词原形
 
与过去事实相反
 if + 主语 + 动词过去完成式
 主语 + would/could/might/should + have + 过去分词
 
与将来事实相反
 ①if + 主语 + 动词过去式
②if + 主语 + were to + 动词原形
③if + 主语 + should + 动词原形
 主语 + would/could/might/should + 动词原形
 
此表中需要注意几点：
 动词在表示与现在事实相反的从句中一般用were。
  在非正式情况下,第一、第三人称后偶尔也用was。如:
If he were you,  he would go at once。 
2．主句中的should通常用于第一人称，would,  could以及night可以用于各种人称。
3．在表示与将来事实相反的条件句中，只能用should，而不能用would,  could和might等。
  如：
If it should rain tomorrow,  we would not go camping。 （should rain也可以用rained,  were to rain代替）
4．当条件状语从句表示的行为和主句表示的行为所发生的时间不一致时，被称为“错综时间条件句”，动词的形式要分别根据它所表示的不同时间作相应的调整。
  如：
If you had followed my advice,  you would be better now。 
（二）含蓄条件句
有时为了表达的需要，在虚拟语气中并不总是出现if引导的条件句，而通过其他手段来替代条件句。
1．用介词短语替代条件句。
  常用介词有with,  without,  but for等。如：
We couldn’t have succeeded without your help（=If we hadn’t got your help）。 
But for the rain（=If it hadn’t been for the rain）,  we would have finished the work。
   
2．用不定式短语来替代条件句。如：
It would be only partly right to do it in this way（=If we do it in this way）
3。 用定语从句代替条件句。如：A nation which stopped working would be dead in a fortnight（=If a nation stopped working,  it would be dead in a fortnight）。
   
4．用一个副词或连词表示虚拟条件句。如：
He telephoned to inform me of your birthday。  or (=If he hadn’t telephoned to inform me of your birthday)I would have known nothing about it。
   
5．用分词短语或独立主格结构来替代条件句。如：
Everything taken into consideration（=If everything had been taken into consideration），they would have raised their output quickly。
   
 
（三）常用虚拟语气的句型
在下列句型中，常用“should + 动词原形”且should可以省略：
1．在动词demand,  order,  require,  insist,  suggest等后that引导的宾语从句中。
  如：
The doctor suggested/insisted that she(should) stay in bed for a few days。 
注意：当suggest作“暗示、表明”讲，insist作“坚持说”讲，即坚持事实是怎样时，后边that从句不用虚拟语气。
  如：
Mike insisted that he had never stolen anything。 
The look on his face suggested that he was angry。 
2．在名词advice,  order,  suggestion,  request等后边that引导的同位语从句，或这些名词作主语时后面that引导的表语从句中。
  如：
We all agreed to his suggestion（that）we（should）give up the plan。 
My idea is that he（should）accept the job。 
3．在It is + 形容词(necessary,  natural,  strange,  etc。
   ) + that从句中。如：
It’s necessary/important/natural that we (should) clean the room every day。 
4．在It is/was a pity（a shame,  no wonder,  etc。
   ） + that从句中。如：
It was a pity/a shame/no wonder that our team should lose the game。 
当然上面3、4两类的that从句中也可以不用虚拟语气，而用陈述语气。
  如：
It is strange that he did not come yesterday。 
It is a pity that you can’t dance。 
 
（四）It is（high/about）time（that）…句式中常用动词的过去式。
  
此句型表示“该……”，用来表示提议。如：
It is high time we started out。 
 
（五）用动词的过去式表示与现在事实（或将来事实）相反，用动词的过去完成式表示与过去事实相反的句型。
1．wish后that引导的宾语从句中。
  如：
I wish I were as clever as you。 
I wish he would try again。 （表示与将来事实相反）
 if(though)引导的表语从句或方式状语从句中。如:
He looks as if/though he were an artist。
   
3．would rather后的宾语从句中。如：
I’d rather you didn’t go there。 
4．If only引导的感叹句中。如：
If only引导的感叹句中。如：
If only I were a bird。
   （我要是只鸟就好了。）
5．在含有If it were not for或If it had not been for条件句的虚拟语气中。如：
If it were not for the rain,  the crops would/should die。
   
（六）省略if的情况
如果从句中含有系动词、情态动词或助动词时，可省略if，把从句中的were,  had或should提到其主语前形成倒装结构。如：
Were I you,  I wouldn’t do that。 
Had you come earlier,  you would have seen him。
   
Should it rain tomorrow,  I would stay at home。 
(七)虚拟语气表示委婉语气的句型
would,  could等词也常被用来表示委婉语气。如：
It would be better for you not to stay up too late。
   
Would you be in enough to show me the way ?
Would you be so kind as to show me the way ?
。