before引导的从句谓语动词到底用什么样的时态呢？详细点，谢了

before 所引导的时间状语从句 所表示的动作 发生在主句动作之后 如果强调主句的动作发生在前 主句谓语 要用过去完成时 ；
he had studied english for six years before she went to college.
before 引导的从句用一般过去时 如不强调动作的先后 主句和从句 均用一般过去时 ；
如 she worked in shsnghai before she came here.

主句现在时从句现在完成时
主句一般过去时从句过去完成时

就是从句比主句提前一个时态

也有
主句现在时从句一般过去时
主句一般过去时从句过去完成时
这种情况

不能一概而论

it is time since sb did
it was time before sb did(since sb had done)
it will be time before sb dose
中学阶段掌握这些就够，简单好记，不会错。

主句现在时从句现在完成时,主句一般过去时从句过去完成时it is time since sb did
it was time before sb did(since sb had done)
it will be time before sb dose