虚拟语气是一种动词形式，表示说话人的一种愿望，假设，怀疑，猜测， 建议等含义，虚拟语气所表示的含义不是客观存在的事实。
If I were you, I'd take them away.
如果我是你的话，我就会带走他们。
If I had met Li Hua, I could had told him.
如果我碰见了李华，我就告诉了他了。
If I had time. I could come to help you.
如果我有时间，我会来帮助你的。
He suggested that our class should be divided into five groups.
他建议说我们的班应该分成五个小组。
He speaks to us as if he had been there.
他给我们讲的好象他去过那儿。
虚拟语气的用法
1) 虚拟语气用在简单句中，表示祝愿，命令。
May you be happy.
祝你幸福。
May you have a good time.
祝愿你玩的痛快。
May the friendship between us last long.
祝愿我们的友情天长地久。
Have a good journey!
祝愿你旅途愉快！
You go out！
你出去！
2) 虚拟语气用在宾语从句中。动词 wish, suggest,order,insist, propose,等词后面的宾语从句表示的是一种虚拟语气， 宾语从句中的动词动作表示的只是一种愿望，要求。
I wish she would be on my side.
我希望她能站在我一边。
I wish I could help him.
我希望我能帮助他。
He insisted that all of us should be there on time by any means.
他坚持要我们大家想尽办法按时去那儿。
动词 demand, suggest, order, insist, propose 后面的从句中，"should" 可以省略。
The teacher suggested that we (should) clean the blackboard after class.
老师建议我们课后把黑板擦了。
He ordered that the students wash the clothes every week by themselves.
他要求学生每周都要自己洗衣服。
3) 虚拟语气用在主语从句中。
在句型 "It is important (necessary, strange, natural) 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.
我们有必要出去散散步。
It's natural that she should do so.
她这样做是很自然的。
It's important that we should take good care of the patient.
重要的是我们要照顾好病人。
4) 虚拟语气用在状语从句中。
虚拟语气最多地用在表示条件的状语从句和表示结果的主句中。在表示与事实相反的虚拟语气时，动词有三种时态形式，即现在，过去和将来。
与现在事实相反的：
条件从句 结果从句
If I (we,you,he,they)+ 动词过去式.. if I (he,she) were... I (we) should + 动词原形。 He (you,they) would + 动词原形。
与过去事实相反的：
条件从句 结果从句
If I(we,you,he,they)+ had + 过去分词 I(we)should + have+ 过去分词。He (you,they) would + have + 过去分词。
If I were you, I should buy it.
如果我是你，我就买了它。
If I had time, I would study French.
如果我有时间，我会学习法语的。
If she knew English,she would not ask me for help.
如果她懂英语的话，她不会找我帮忙的。
If you had got up earlier, you could have caught the train.
如果你早一点起床，就会赶上火车的。
If it were fine tomorrow,I would go shopping.
如果明天天气好，我就去买东西。
有关虚拟语气的几个问题：
1) 有时if引导的状语从句可以省略 if，而把从句中的动词 were, had 或 should 移到主语前面。
Were she younger, she would do it .
如果她年青点， 她就会干的。
Had he known her address, he would had gone to visite her.
如果他知道她的地址，他会去看她的。
2) 有时表示虚拟语气的条件从句或者主句都可以省略，而只剩下一个主句或者一个条件从句。
I could help you.
我本来可以帮助你。
If I had time.
我要有时间该多好啊。
She should have come to the party.
她应该来参加聚会。
If he had much more money.
如果他有更多的钱就能...。
3) 虚拟语气中，条件从句的动词动作可以和主句的动词动作时态不一致。
If they had studied hard, they could do it easily now.
如果他们以前努力学习的话，现在干的就会容易些。
If he had not taken my advice, he wouldn't do it much better like this.
如果他不听取我的建议，他就不会干得这样好
参考资料：http://www.cnread.net/cnread1/yyxx/yyyf/013.htm