before引导的主句过去式,从句应用什么时态

应该是用过去完成时,
因为在含有before的主从复合句中,一般在从句中使用一般过去时,主句中使用过去完成时.例如：
Lily had written some letters before she went out shopping.
有时,也可以在从句中用过去完成时,主句中用一般过去时,这样就可使描述更为生动.试比较：
We got to the station before the train had left.火车尚未开动之前,我们就到了车站.
We had got to the station before the train left.火车开出之前,我们就已经到了车站.