1.概念：表示在过去某一时间对将来某一时刻以前所会发生的动作，常用在虚拟语气中，表示与过去的事实相反。

2.基本结构：should/would have done sth.

3.例句：

I thought you’d have left by this time.我想这会儿你已经走了。

He told them he would have finished it by 8 o’clock.他告诉他们他会在8点以前干完。