1、 if 引导的条件状语从句可以分为两类：真实条件句（ Sentences of Real Condition）和虚拟条件句。凡是假设的情况发生性可能很大，就是真实条件句；当假设是不大可能实现时，就是虚拟条件句。如：If I have time, I will help you with this work. 如果我有时间 , 我会帮助你做的。If time permits, well go fishing together. 如果有时间的话，我们就一起去钓鱼。If I were you , I would have attended the meeting.我要是你的话，就去参加会议了。If
2、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。if 在条件状语从句中的虚拟语气：1. 表示与现在事实相反的情况。其句子结构为：从句：if + 主语+ 动词的过去式(be 用 were) + 主句：主语+ would (should, could , might) + 动词原形+ If I were you, I would read it again. 如果我是你的话，我再读一遍。（事实上我不是你）If he hurried, he could catch the first
3、 bus. 他要是快点能够赶上头班公共汽车。（可他不急）If she knew English, she would not ask me for help. 如果她懂英文，她就不必要我帮了。2. 表示与过去事实相反的虚拟条件句。其句子结构为：从句：If + 主语+ had +过去完成式动词+ 主句：主语+ would (should, could, might) + have +过去完成式动词+If you had taken my advice, you would not have made such a mistake.如果你听了我的劝告,就不会犯这样的错误。（事实上你没有听我的劝告）
4、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。If I hadnt been ill yesterday, I might have come to school.昨天要是不生病，我是可能来上学的。（事实上我生病了）If you had studied harder last term, you could have passed the exam. 如果你在上个学期用功一些，你就会在考试中过关了。3. 表示与将来事实相反的虚拟条件句 (对将来的事实实现的可能性不大)。其句子结构为：从句：If + 主语+ should (或 were) + 动词原形+ 主句：主语+ would (could, should, might) + 动词原形+ If it should rain tomorrow, I would stay at home. 如果明天下雨的话，我将待在家里。（根据天气情况，明天不可能下雨）If he came tomorrow, I would do it with him. 如果明天他来的话，我将和他一起做此事。（事实上他来的可能性很小）If he were here, I would give him the books. 如果他在这儿，我可能会把书给他。
