Work on it had begun befor my sister left.请问这是什么从句?这个句子的引导词，主语，从句分别是什么？

work on it had begun 为啥不用被动? work on it had begun before my sister left... Work on it had begun before my sister left.分析句型Work on it had begun before my sister left.分析句型并翻译 请问“work on it had begun before my sister laft.”一句中的work on it怎么讲,如题. 这个句子为什么要用过去完成时?Work on it had begun before my sisiter left. work on it had begun before my sister left(这是什么句型? 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.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 为什么可以放在句首?例如：Work on it had begun before 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 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.这句话的主语是倒装句吗? Work on it had begun before my sister left 请帮我拆分一下这个句子的成分. 关于Work on it的疑问..Work on it had begun before my sister left.Work是主语..那么在这里Work on it是作为一个整体理解吗? work on it had begun befor my sister left.这句话的主语是什么?work 后为什么要加on.主要就是不明白work on it的句子成分,希望高手给讲讲 Work on it had begun before my sister left.work on it 是什么结构We are now living in a beautiful new house in the country.Work on it had begun before my sister left.我们现在住在乡间的一栋漂亮的新住宅里.这栋房子在我姐姐 Work on it had begun before my sister left.Work前为什么不加theWork on it had begun before my sister left.这幢房子是在我姐姐离开前动工的.(23课)By the end of next year,they will have finished work on the new stadium.到明年年