​为何会存在主从句都用过去完成时的状况

He had waiteduntil she had finished her work.

He had waiteduntil she finished her work.

以上二个句子有细微差别。假定she完成工作的时间为五点十分。第一句表示他等到五点十五分左右，见到了她，等待的动作结束。

第二句则表示他等到她五点十分干完活，结束等待。

第一句之所以用二个过去完成时，是由于说话人心目中关注的时间是五点十五分他见到她这个时间，而等待wait和finish这二个动作都是在五点十五分之前发生的。所以主从句都用过去完成时。但事实上，until从句用一般体和完成体的时间差异非常小，一般让人们忽视，因此一般在until从句用完成体较少。有些语法书甚至觉得用完成体和一般体意义相同。这是忽视细微的时间差别。

以下四个句子意义相差不大，或者说可以忽视他们之间的差别：



He had waiteduntil she had finished her work.

He had waiteduntil she finished her work.

He waiteduntil she had finished her work.

He waiteduntil she finished her work.