Question

would have done有什么用？

Answer 1

would have done 在语法时态上称作“过去将来完成时。”
语法教科书上对这个时态的解释是“表示从过去观点看将来某时某动作已完成”，并注明“这个时态用得相对比较少。”例句：
I thought Sophia would have told you something. 我想索菲娅会已告诉你一些情况。
He knew by the time he arrived she would have gone home. 他知道他到时她会已经回家。
实际上，这个所谓的过去将来时很常见，只是不以“时态”为主，而是以“语态”为要 —— 虚拟语态。would have done 是很常用的假设表达方式，用在对已经发生的事表达不可能发生的看法（假设）。比如，车祸已经发生。在谈论时常会表达希望此事不曾发生：
I'm sorry for the loss of the ambassador. I wish this would not have happened. 我为失去这位大使感到惋惜，真希望这件事不曾发生。
The accident would not have happened if the airplane had arrived on time. 假如这趟飞机正点到达，这一车祸或许不会发生。