Use ago to say how much time has passed from the time something happened to the present time 用ago表示从事情发生到现在时间过去了多久
We went to Madrid two years ago.我们两年前去了马德里。
Use before to say how much time passed from the time something happened to a time in the past 用before表示从事情发生到过去某个时间是多久
Her husband had died many years before.那时她丈夫已去世多年。
Previously is used in the same way, but is more formal previously的用法相同,但更加正式
The meeting was a follow-up to one that had been held four days previously.这次会议是四天前那次会议的后续。
Use the past tense, not the present perfect, with ago ago和过去时连用,不和现在完成时连用
I started (NOT 不说 I've started) a new job a few weeks ago.几星期前我开始了新工作。
Do not use a preposition ('at', 'in', 'on' etc) before a phrase with ago 带ago的短语之前不用at,in,on等介词
They first met fifteen years ago (NOT 不说 at/in fifteen years ago).他们15年前相识。
Do not use 'since' or 'before' with ago ago不和since或before连用
I came to the US two months ago (NOT 不说 since/before two months ago).我两个月前来到美国。