If had done,sb would do sth和If had done,sb would have done有什

前者是说“如果某事发生了,某人接下来就会做什么.” 有了前者,就会有后者,表前因后果关系
If you had passed the exam,your mom would buy you a new car.
后者是说“如果某事发生了,某人就应该已经做了什么了.”有了前者,后者就应该已经做了什么了（而实际上后者并没有做）,这是一种事后诸葛亮的说法
If you had passed the exam,your mom would have brought you a new car already.换句话说,你没考过,否则你妈妈早就帮你买好那个车了（考试已经失败,车也没买）
:D