在以主语+had done, before/by+过去时间状语词组/从句中的运用：
Mr.Wanghad attended the conference and delivered a speech before he visited Yorkshire.
王先生在去约克郡之前已经出席了会议并发表演讲。（before引导的从句，一般过去时）
Cappuccino had become a quick seller in Starbucks,by the end of last month.
到上个月底为止，卡布奇诺已经成为了星巴克的畅销品。（by引导的时间状语）
温馨提示：
