Had I receivd your letter,I would have written back two days ago.为什么要用Had I
问题描述：
Had I receivd your letter,I would have written back two days ago.
为什么要用Had I
答
过去完成时疑问句啊
答
虚拟语气had提前，本来应该是if I had received
had提前加重点语气
答
一种特殊的虚拟语气表达方法.if 省略 had 提前,从而达到加强语气的目的.
又如：
Had I known you were coming today,I'd have met you at the railway station.
原句:If I had knownyou were coming today,I'd have met you at the railway station.
答
虚拟语气
翻译：如果我收到你的信，2年前我就会给你回信了