标志性词语
过去完成时常见的标志性时间状语主要有：
1、by the end of+过去时间
By the end of last year, we had solved more than 100 problems. （到去年年底，我们已经解决了100多个问题。）
2、by the time+从句（动作发生在过去）
By the time she left the city, she had used up all her savings. （到她离开这个城市的时候，她的存款已全部用完。）
3、by (the time of)+过去时间
He had taught maths for four years by last July. （到上个七月为止，他已经教了四年数学了。）
4、before+过去时间
He had never ridden a horse before last month. （到上个月以前他从来没有骑过马。）
