Ago follows a word or expression referring to a period of time. It is usually used with a past tense, not with a perfect tense 单词ago置于表示一段时间的词或表达法之后,通常与过去时连用,而不与完成时连用。
used for saying how much time has passed since something happened以前;从前
Use ago to say how long before the present time something happened: 用ago 表示某事是在距现在之前多久发生的: He died two years ago.
Use before to say how long before a time in the past something had happened: 用before 表示某事是在过去某时间之前多久发生的: I remembered that I had met her ten years before.
Use for to say how long something in the past continued: 用for 表示过去的某事延续了多长时间: Their marriage lasted for fifteen years.