You only use ago when you are talking about a period of time measured back from the present. 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 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.

ago仅用于谈及现在某个时间以前的时间。如果谈论过去某时间之前的时间，则用before或previously：He had died a month before（他一个月之前就死了），She had rented the flat some fourteen months previously（她大约14个月前租下了该公寓）。for用于表示过去、现在或将来持续的某段时间，或表示某事多长时间没有发生：She slept for eight hours (她睡了8个小时），He will be away for three weeks（他将离开3个星期），I hadn't seen him for four years (我有4年没见过他了）。表示某段时间开始用since：She has been with the group since it began（该集团创立之初她就在那儿工作），the first civilian president since the coup 17 years ago（自17年前发生政变以来的第一任平民总统）。since也可表示某事最后一次发生的时间或某事多长时间没有发生：She hadn't eaten since breakfast（她早饭后就一直没吃过东西），It was a long time since she had been to church（她已经有很长时间没去教堂了）。