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。
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。
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。
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。
