I have been an engineer for more than ten years,but I was once a _英语解答

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?

人气：254 ℃　时间：2019-11-04 22:24:59

解答

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*******