If you had asked him yesterday, you would know what to do now. 如果你昨天问过他,今天就知道做什么了。
(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)。如果昨晚下过雨,今天就会很冷了。