如果从句和主句都表示过去的,那从句用had done,主句用would ,could,might,should+have done
如：If you had taken my advice,you would not have been late for the meeting yesterday.如果从句表示过去的,但主句表示现在的,从句用had done,主句用would ,could,might,should+do,如：If you had taken my advice,you would not be late for the meeting now.这种一般都有个时间,如果没有表示时间的词也可以从上下文看出.