概念
虚拟语气用来表示说话人的主观愿望或假想，所说的是一个条件，不一定是事实，或与事实相反。
2） 在条件句中的应用
条件句可分为两类，一类为真实条件句，一类为非真实条件句。非真实条件句表示的是假设的或实际可能性不大的情况，故采用虚拟语气。
这是虚拟语气的概念。 我找了个网址。因为举例很多，所以造成了字数超限。你可以转到http://www.bjmti.com/resource/ShowArticle.asp?ArticleID=3647希望能帮你。
参考资料：http://www.bjmti.com/resource/ShowArticle.asp?ArticleID=3647
虚拟语气是一项语法难点，为了让学员们便于理解与记忆，我们把虚拟语气分成三大块来讨论：
1．名词从句中的虚拟语气 标志词+should+动词原形
2．条件从句中的虚拟语气 ①三种基本态 ②倒装虚拟句 ③混时虚拟句 ④含蓄虚拟句 ⑤跳层虚拟句
3．其它情形中的虚拟语气（wish/would rather/It’s high time…/If only…/lest）
一．三种基本形态（1）
表虚拟的时间 if从句谓语形式 主句谓语形式
现在： did / were would (should, might, could)+do
过去： had done/ had been would (should/might/could)+have done(been)
将来： were to/ should+do would (should, might, could)+do
三种基本形态（2）（这是基础，建议牢记下面的例句，争取能脱口而出）
与现在事实相反：If I had enough money, I would buy a book.
与过去事实相反：If I had had enough money, I would have bought a book.
与将来事实相反：If I were to have enough money, I would buy a book.
If I should have enough money, I would buy a book.
二、倒装虚拟句（只能与过去/将来事实相反，省去if，只有三种倒装形式）
Had I had enough money, I would have bought a book.
Were I to have enough money, I would buy a book.
Should I have enough money, I would buy a book.
三、混合时间虚拟语气（主要是抓住时间状语）
(对过去事实虚拟+对现在事实虚拟)
If I had studied English at school, I could read the English novel now.
(对现在事实虚拟+对过去事实虚拟)
If she were not so careless, she wouldn’t have made such a mistake.
四、含蓄虚拟句（在四级考试中目前还未出现此考点，但在阅读中常见）
3. 虚拟语气用在主语从句中。在句型“It is important (necessary, strange, natural等) that...” 中，that 后面的从句中的谓语动词用(should) + 动词原形。
It's necessary that we should have a walk now. 我们现在有必要出去散散步。
What would I have done without you?
But for their help, I would not have finished the task.
But that the doctor arrived on time that day, they would have been dead.
Given more time, I would have been able to finish the test.
五、跳层虚拟句（即句子一半为虚拟，另一半为陈述语气；but前句用虚拟而or/otherwise后句用虚拟） (but)
He would put on weight, but he doesn't eat much. (与现在事实相反)
He would have put on weight, but he didn't eat much. (与过去事实相反)
(or, or else, otherwise)
I forget where I read the article, or I would show it to you now. (与现在事实相反)
Mary couldn't have received my letter, otherwise she would have replied before now. (与过去事实相反)
参考资料：http://cidian.sina.com.cn/b/3324579.html
