1 条件句中的虚拟语气：从句中提出一种与客观现实不相符或根本不可能存在的条件，主句会产生的一种不可能获得的结果。条件句中的虚拟语气根据不同的时间有三种不同的形式。
(1) 将来时的条件句中的虚拟语气（should 动词原形或were to 动词原形）。如：
If he should go to Qing Hua University, he would make full use of his time. 如果他要上清华大学的话，他就会充分利用他的时间了。
If he were to come here, he would tell us about it. 如果他要来的话，他会通知我们一声。
(2) 现在时的条件句中的虚拟语气。（动词过去式(be 用 were)或would / should / might / could 动词原形）如：
If he were free, he would help us. 要是他有空的话，它会帮助我们的。
If he studied at this school, he would know you well. 如果他在这所学校学习的话，它会对你很熟悉。
(3) 过去时的条件句中的虚拟语气。（had 动词过去分词或would / should / might / could have 动词过去分词）如：
If I had seen the film, I would have told you about it. 我如果看过这场电影，我会把电影内容告诉你了。
If I had got there earlier, I would have met Mr. Li. 如果我早点到那儿，我就会会到了李先生。
2 运用条件句中的虚拟语气时，须注意的几个问题
(1) 当从句的主语为第三人称单数时，谓语动词若是系动词be时，可用 was 代替 were。但在倒装虚拟结构及 if I were you, as it were 中，只能用 were。如：
Were I ten years younger, I would study abroad. 要是我还年轻十岁的话，我会去国外学习。
If I were you, I would try my best to grasp the chance. 要是我是你的话，我要尽力抓住这次机会。
(2)
有时，虚拟条件句中，主、从句的动作若不是同时发生时，须区别对待。
①从句的动作与过去事实相反，而主句的动作与现在或现在正在发生的事实不符。如：
If I had worked hard at school, I would be an engineer, too. 如果我在学校学习刻苦的话，我现在也会使工程师了
If they had informed us, we would not come here now. 如果他们通知过我们的话，我们现在就不会来这里了。
②从句的动作与现在事实相反，而主句的动作与过去事实不符。如：
If he were free today, we would have sent him to Beijing. 如果他今天有空的话，我们会已经派他去北京了。
If he knew her, he would have greeted her. 要是他认识她的话，他肯定会去问候她了。
③从句的动作与过去发生的情况相反，而主句的动作与现在正在发生的情况相反。如：
If it had not been raining too much, the crops would be growing much better. 如果天不下太多的雨的话，庄家会长得更好。
If he had been working hard, he would be working in the office now. 要是他工作一直努力的话，他现在已进了办公室了。
