虚拟语气中会这样用.
have has +pp have 、has 、had 它们都只是助动词 have has 使后面的动词变成了现在完成式
现在完成时：就是说这个动作已经发生完了,但对现在还有影响
had 是后面的动词变成了过去完成时.我丢了钥匙（我可能进不去屋了）
过去完成时：动作发生在过去的过去.（昨天早上我进办公室前他已经打扫完了卫生）
在虚拟语气中：
与过去事实相反的假设时：条件句 过去完成时
主句 过去将来完成时
If I had used that word to describe you,it would have been better.
If I had eaten the rice,I would have gone to school with you.