形式
should / would have done sth. 　　

用法 　　
1、 过去将来完成时表示在过去将来某一时间以前发生的动作，并往往会对过去将来某一时间产生影响。
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。 　　
I guessed that Helen would have told her something. 我猜海伦会告诉她一些情况的。 　　

2、 过去将来完成时还常用在虚拟语气中，表示与过去的事实相反。