Before是表示过去，应用于过去完成时态。过去完成时1. 概念：表示以过去某个时间为标准，在此以前发生的动作或行为，或在过去某动作之前的行为，即“过去的过去”。2. 常见时间状语标志：before, by the end of last term / week / month / year 等。例如：Most of the guests had left when he arrived at the party. 当他到达晚会现场时，大多数客人已经离开了。By the 1990s the number had fallen to about 50,000. 截止到20世纪90年代， (藏羚羊的) 数量下降到了大约5万只。 Before he volunteered to direct the traffic, Timoteo had had lots of jobs. 铁穆特欧在志愿指挥交通之前做过很多工作。before long与long before的区别Before long, he went to America. 不久，他就去了美国。I hope to see you before long. 偶希望不久后再见到你。She said she had read the novel long before. 她说她很久以前就读过这部小说。That happened long before. 那件事发生在很久以前。
