虚拟语气if we had done结构是能加确定的过去时间状语。

If you had come here five minutes ago,you would have seen Tom.

If you had studied before, you would have passed the exam.

我觉得你不该提这么简单的问题。

if sb had done sth,... 如果这是虚拟语气的话，就表示与过去的事实相反。而过去的事实可能用一般过去时或现在完成时表示，前者通常有确切的过去时间状语，后者则没有。二者改为if虚拟条件句时，一般过去时和现在完成时都改为了had done, 一个带有确切时间状语，一个不带。这难道不简单吗？