1、if引导的条件状语从句中的虚拟语气
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。
一、if引导的条件状语从句的分类及虚拟条件句的判断
1、可以把条件句分为两类：
1).真实条件句（Sentences of Real Condition）：凡是假设的情况发生性可能很大，就是真实条件句。例如：
⑴、If I have time , I will help you with this work.如果我有时间会帮助你做此工作的。
(2)、If time permits, we'll go fishing together.(如果有时间的话，我们就一起去钓鱼。）
2).
2、虚拟条件句（Sentences of Unreal Condition）：当假设是不大可能实现时，就是虚拟条件句。例如：
⑴、If I were you , I would have attended the meeting. 如果我是你的话，就去参加会议了。
⑵、If he had come here yesterday, he would have seen his old friend.假如他昨天来这儿的话，就会看见他的老朋友。
（3）、If it had rained yesterday, we would have stayed at home.（如果昨天下雨的话，我们就会留在
3、家里。）
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
4、
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：
⑴、If I had come her yesterday, I would have seen him.
⑵、If I were a teacher, I would be strict with my students.
⑶、If it should snow tomorrow, they couldn't go out.
一、与过去事实相反
连接词
条件从句
结果从
5、句
If
had+过去分词
Should
Would + have+过去分词
could
might
二、与现在事实相反
连接词
条件从句
结果从句
If
1.动词过去式(或were)
2.助动词（过去式）+动词原形
Should
Would +动词原形
could
might
三、与将来事实相反
连接词
条件从句
结果从句
If
1.should+动词原形
2.动词过去式
3．were to+动词原形
Should
Would +动词原形
could
might
4、注意事项
①如果从句中含有were/ s
6、hould/ had时，则可以把这三个词置于句首，省略if，并在从句中采用倒装语序。
（1）If she were here, she would agree with us. →Were she here, she would agree with us.
（2）If it should happen, what would you do? →Should it happen, what would you do?
（3）If he had recognized me, he would have come over. →Had he recognized me, he would have come over.
②在现代英语中if条件状语从句中的谓语动词如果是be动词，其过去形式一般用were。
5. 总结
以上3大类情况属于if引导的非真实条件句的虚拟语气的常见基本形式，但其所有形式绝不仅限于此。如：
If you had followed my advice, you would be better now. 这显然是假设条件与过去相反，而主句表达现在情况的混合虚拟语气句，并且没有任何语法错误。因此需要说明的是，要学好虚拟语气，对句意的理解才是根本。
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