work on it had begun 为啥不用被动?

work on it had begun 为啥不用被动? work on it had begun befor my sister left. Work on it had begun before my sister left. work on it had begun before my sister left... work on it had begun before she left.这里为什么不用worked 不是过去发生的嘛 Work on it had begun before my sister left.work on it 是什么结构 请问“work on it had begun before my sister laft.”一句中的work on i Work on it had begun before my sister left.Work前为什么不加the Work on it had begun before my sister left.分析句型 Work on it had begun before my sister left.是时间状语从句吗? 请问work on it had begun before my sister left 是什么句型 work on it had begun before my sister left(这是什么句型?