If you are talking about a period measured back from some earlier time, you use before or previously. He had died a month before…She had rented the flat some fourteen months previously. You only use ago when you are talking about a period of time measured back from the present. You use for to say how long a period lasts in the past, present, or future, or how much time passes without something happening. She slept for eight hours… He will be away for three weeks… I hadn't seen him for four years. You use since to say when a period of time started. She has been with the group since it began. …the first civilian president since the coup 17 years ago. You also use since to refer to the last time that something happened, or to how much time passes without something happening. She hadn't eaten since breakfast… It was a long time since she had been to church.

如果谈论过去某个时间以前的时间，用 before 或者 previously：He had died a month before（他一个月前去世了），She had rented the flat some fourteen months previously（她大约14个月以前租下了这套公寓）。ago仅用于谈论现在之前的某段时间。for用来表示过去、现在或将来持续的一段时间或者某事多长时间没有发生，如：She slept for eight hours（她睡了8个小时），He will be away for three weeks（他会离开3个星期），I hadn't seen him for four years（我已经有4年没有见过他了）。since用来表示某段时间的开始，如：She has been with the group since it began（自从该集团创立起她就一直在其中工作），the first civilian president since the coup 17 years ago（自从17年前发生政变以来的第一位平民总统），since也可以用来某事最后一次发生的时间或者某事多长时间没有发生，如：She hadn't eaten since breakfast（她早餐以后就没吃过东西），It was a long time since she had been to church（她好久没去过教堂了）。