before 引导的是个时间状语从句.
它引导的从句时态有主要有二：
1.主句是一般将来时,before 引导的从句要用一般现在时,如：
I will make an arrangement before he comes.
2.before 引导的从句用一般过去时,主句用过去完成时.
The thief had run away before the police came.