Work on it had begun before my sister left.这栋房子在我姐姐离开之前就已动工了.
问题描述：
Work on it had begun before my sister left.
这栋房子在我姐姐离开之前就已动工了.
答
work on 是定语，it 是主语，hadbegun 是谓语，before是宾语
答
过去完成时。
work （工作）是主语；on it 状语，修饰work（工作）；had begun，早已开始；
before my sister left 为时间状语。
答
work on it 为主语部分 谓语是had.before my sister left 为时间状语.