16.3 混合条件句
 
  主句与从句的动作发生在不同的时间,这时主,从句谓语动词的虚拟语气形式因时间不同而不同,这叫做混合条件句。
  If you had asked him yesterday, you would know what to do now.
  (从句与过去事实相反,主句与现在事实相反。)
  If it had rained last night (过去), it would be very cold today (现在).