if引导的从句什么时候用虚拟语气

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.
⑵、As long as I say anything wrong,you must point it out.只要我说了什么错话,你一定要指出来.
⑶、If time permits,we'll go fishing together.(如果有时间的话,我们就一起去钓鱼.）
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.
⑴、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.
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.
4、注意事项
①if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.