work on 为什么可以放在句首?例如：Work on it had begun before my sister l

work on 为什么可以放在句首?例如：Work on it had begun before my sister l 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 on it had begun before my sister left... 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.work on it 是什么结构 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 before my sister left.这句话的主语是倒装句吗? 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?