had been doing 这个过去完成进行式 用在表达在过去的某段时间的某个动作,而那某段时间的动作和现在没有关系了
因为如果要表达的,和现在还有关系,就要用现在完成时态
he had been working as a teacher in Beijing before he came back home
在他回来家乡之前,他一直在北京当老师