1.虚拟语气在假设过去情况的条件句中的用法
在表示对过去情况假设的非真实条件句中，从句应用过去完成时，即从句的谓语动词应用“had+过去分词”的形式，主句的谓语动词应用“should/would/could/might have+过去分词”的形式。
If I had been a little more careful, I could have found the data error.
如果我更认真一点，我就能发现这个数据错误了。
If we hadn't made adequate preparations,the conference wouldn't have been so successful.
如果我们没有做好充分的准备工作，会议就不会如此成功。
2.虚拟语气在假设现在情况的条件句中的用法
在表示对现在情况假设的非真实条件句中，从句的谓语动词应用过去式(be动词用were)，主句的谓语动词应用“should/would/could/might+动词原形”的形式。
If we had a map in hand now, we would easily walk out of the forest.
如果现在我们手头有地图，我们就会毫不费力地走出森林了。
If you gave up now, you could miss a chance to change yourself.
如果你现在放弃，你就会错过一个改变你自己的机会。
3.虚拟语气在假设将来情况的条件句中的用法
在表示对将来情况假设的非真实条件句中，从句的谓语动词应用过去式(be动词用were)或者“shouldwere to+动词原形”的形式，主句的谓语动词应用“should/would/could/might+动词原形”的形式。
If I took/should take/were to take over the work next week,I would do it in a different way.
要是我下周接手这项工作，我会以不同的方式去做。
If there were/should be/were to be anything wrong in the future,they would tell you at once.
如果未来有什么问题，他们会马上告诉你的。
4虚拟语气在省略if的非真实条件句中的用法
在非真实条件句中，若从句中含有were,had或should，可以省略if，并把这些词提至主语之前，用倒装结构，以强调假设条件的虚拟性或突出说话人的主观愿望。
Were I you, I would directly refuse his unreasonable demand.
如果我是你，我就会直接拒绝他的不合理要求。
Had she known the truth, she would have forgiven his behavior.
要是她知道真相的话，她就会原谅他的行为。
Should it rain tomorrow, I would stay at home and read some books.
如果明天下雨，我就会待在家里看一些书。
5.虚拟语气在错综时间条件句中的用法
在非真实条件句中，主句和从句所指的时间在一般情况下是一致的，但有时主句谓语动词表示的动作和从句谓语动词表示的动作发生的时间不一致，这时主句和从句谓语动词的形式应根据句意和各自的时间状语来确定。
If they had left home in the early morning, they would arrive in half an hour. (从句是对过去情况的假设，主句是对将来情况的假设)
要是他们一大早就离开家了的话，他们在半个小时后就会到了。
If you hesitated at this moment, you might lose a good chance in the future. (从句是对现在情况的假设，主句是对将来情况的假设)
如果你此刻犹豫不决的话，你将来可能会失去一个好机会。
If they had ever cared more about the child, he wouldn't let them down now.(从句是对过去情况的假设，主句是对现在情况的假设)
要是他们曾更多地关心这个孩子，他现在就不会让他们失望了。
非真实条件句中的主句通常是陈述句，但也可以是疑问句、感叹句或祈使句。如：
If she had everything, would she be happy?
如果她拥有一切，她会幸福吗？
If he had married Alice,how happy he would have been!
他要是娶了爱丽丝的话，他会多么幸福啊！
若省略i的非真实条件句为否定句，否定词not仍应置于主语之后，不能与were,had或should一起提前，也不能缩写成weren't,hadn't或shouldn't后再置于句首。如：
Were I not so busy,I would go shopping with you.
如果我没有那么忙，我就和你一起去购物了。