Work on it had begun before my sister left.分析句型

Work on it had begun before my sister left.分析句型 Work on it had begun before my sister left. work on it had begun before my sister left... 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? Work on it had begun before my sister left.work on it 是什么结构 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.是时间状语从句吗? work on it had begun before my sister *left*(离开)和I am one of Work on it had begun before my sister left.这句语法讲解下,有点迷糊. work on it had begun befor my sister left. Work on it had begun before my sister left中的WORK 不是动名词作主语 要加