before用作“时间副词”，主要有两种用法：

1. 以前；以往；过去： If someone has done something before, they have done it on a previous occasion. If someone has not done something before, they have never done it.  这时，句子多用现在完成时或一般过去时。

I've been here before.以前来过这儿。

I met Professor Lown before.以前见过劳恩教授。

2. 在（某时间或某事）以前： You use before when you are talking about time. For example, if something happened the day before a particular date or event, it happened during the previous day.  这时，句子多用过去完成时（即表达“过去的过去”）。

The war had ended only a month or so before. 战争大约一个月前才刚刚结束。

（说明动作发生在另一个过去时间的一个月前。比较：The war ended only a month or so ago.）

They had left here three days before.他们早在三天前就离开了。（说明动作发生在另一个过去时间的三天前。比较：They left here three days ago.）