一、概念
中文译作"虚拟语气".它是一种动词形式,表示说话人的某种假设、愿望、怀疑、猜测、建议等含义.
二、语法结构
1.if结构（非真实条件句——表示的是假设的或实际可能性不大的情况）
与..事实相反 If从句 主句
过去 Had done Would* have done
现在 Were/did Would* do
将来 Should do/were/were to do Would* do
例句：If there hadn’t been much rain in spring, we would have had a good harvest now. [过去]
If he smoked less, his cough might be well soon. [现在]
If the lecturer should be late, you would have to make a speech first. [将来]
备注：
（1）上表中’*’,would都可转换为should、could、might.
（2) 如果为时间错综句的话,左右两栏可互相搭配,排列组合.
例句：
He would still be alive today if he hadn’t taken that drug.
[与现在事实相反] [与过去事实相反]
(3) if可转换为其他形式
例句：Without your advice, our meeting wouldn’t have been so successfully. [过去]
（=>可转换为：If there hadn’t been your advice, we …）
Your notes are almost illegible (难以辨认的). Notes typed out would be a lot easier to read. [将来]
(=> 可转换为：If the notes had been typed out, it would be a lot easier to read.)
I should have gone to see Dr. Smith and he might have cured me of the disease.[过去]
(=> 可转换为：If he have gone to see Dr. Smith, he ..)
2、wish结构
与..事实相反
过去 Had done
现在 Were/did
将来 Would
备注：可转换为其他形式.
例句：He talks as if (好像) he had done all the work himself. [过去]
I wish I were a bird.
I wish he handn' done that.
I wish I would be rich in the future.
3、should结构
从句中用“should + 动词原形”构成.而且should可以省去.用于此结构的动词有：advise, direct, agree, ask, demand, decide, desire, insist, order, prefer, propose, request, suggest,intend等.
例句：The teacher suggest he (should) read English aloud.
注意：当insist表示“坚持认为”、suggest表示“表明,显示”时,不用虚拟语气.
例句：The look on his face suggested that he was quite satisfied with what I had done for him.
He insisted that he was honest.
4、would rather ＋从句
在这种结构中,从句的谓语动词用过去形式表示虚拟.
例句：I would rather you did this instead of me.
5、主语从句中的虚拟语气
1）It be + 形容词 + that ．．．（should）．．．
用于该句型的形容词是：necessary, good, important, right, wrong, better, natural, proper, funny,
strange, surprising ．
一些名词也可以用于 在该结构中.如：a pity, no wonder．．．．
2）It be + 过去分词 + that ．．．（should）．．．．
用于该结构中的过去分词是表示“建议、请求、命令”等词的过去分词.如：desired, suggested, requested,
ordered, proposed等.
3）It is time（about time, high time）that ．．．（过去式动词形式或should＋动词原形）．．．．
It is high time I went home now. = It is high time I should go home now.
6、表语从句、同位语从句中的虚拟语气
在suggestion, proposal, order, plan, idea, advice, decision等需要有内涵的名词后面的表语从句、同
位语从句中,要使用虚拟语气.其谓语动词应用：should＋原形动词.另外连接从句的that不能省
略.
例：My suggestion is that we should go there at once.
What do you think of his proposal that we should put on a play at the English evening?
三、使用范围及判断
1、虚拟语气表示一种不能实现的假设.该语法主要用于if条件状语从句.也可用于主语从句、表语从句、宾语从句等.
2、if条件状语从句中虚拟语气的判断
判断是真实条件句还是非真实条件句.只有在非真实条件句中才使用虚拟语气.通过句子意思,看假设的条件是否能够实现,能够实现是真实条件句,不能使用虚拟语气；假设的条件不能实现则是非真实条件句,要用虚拟语气.
判断这个假设是与哪个事实相反.通常有三种情况：①与过去事实相反.②与现在事实相反.③与将来事实可能相反.
3、“后退一步法”
后退一步法是指在准确地判断了该句与哪一事实相反后,按虚拟语气的后退一步法处理从句谓语动词的时态.即：在非 真实条件状语从句中,谓语动词按正常情况“后退一步”.也就是：
1）与过去事实相反,在从句中用过去完成时形式表示.
2）与现在事实相反,在从句中用过去一般时形式表示.
3）与将来事实可能相反,在从句中用过去将来时形式表示.
主句中则用情态动词would, should, could 等加一个与从句一致的动词形式.
例：If I had come her yesterday, I would have seen him.
If I were a teacher, I would be strict with my students.
If it should snow tomorrow, they couldn’t go out.
四、注意事项
1.if条件句中如有were, should, had,可以省去if,并使用倒装语序.
2、在现代英语中if条件状与从句中的谓语动词如果是be其过去形式一般用were.
3、wish 后面宾语从句中的虚拟语气,按“后退一步法”处理从句的谓语动词.注意：与哪个事实相反,不能以主句的时态为判断依据,而是根据从句的意义判断.