It took me a long time before I was able to fully appreciate what they had done for me.
问题描述：
It took me a long time before I was able to fully appreciate what they had done for me.
为什么句首的took不用过去完成时,不是过去的过去么、
答
后面had done发生在was able to之前,即过去的过去所以用过去完成时.而took跟was able to发生的时间是同时的所以不用过去完成时.怎么看出是同时的？