A short time before,great trees had covered the countryside for miles around.

题目

A short time before,great trees had covered the countryside for miles around.
为什么用过去完成时
a short time before 如果是过去的时间为什么不用过去时，而要用过去完成时

答案

按句意来理解,看时间状语,a short time before 这个时间说的是过去的时间,所以用过去完成时.
如果时间状语改成Now,就应该用现在完成时了.Now,great trees have covered the countryside for miles around.
对比理解一下.