I have been an engineer for more than ten years,but I was once a worker for three years.

问题描述:

I have been an engineer for more than ten years,but I was once a worker for three years.
这里was once a worker 为什么不是had been a worker?

因为主句用了过去完成时,从句用一般过去时表示曾经做过某一*******