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僅用于談及現(xiàn)在某個(gè)時(shí)間以前的時(shí)間。如果談?wù)撨^去某時(shí)間之前的時(shí)間，則用before或previously：He had died a month before（他一個(gè)月之前就死了），She had rented the flat some fourteen months previously（她大約14個(gè)月前租下了該公寓）。for用于表示過去、現(xiàn)在或?qū)沓掷m(xù)的某段時(shí)間，或表示某事多長時(shí)間沒有發(fā)生：She slept for eight hours (她睡了8個(gè)小時(shí)），He will be away for three weeks（他將離開3個(gè)星期），I hadn't seen him for four years (我有4年沒見過他了）。表示某段時(shí)間開始用since：She has been with the group since it began（該集團(tuán)創(chuàng)立之初她就在那兒工作），the first civilian president since the coup 17 years ago（自17年前發(fā)生政變以來的第一任平民總統(tǒng)）。since也可表示某事最后一次發(fā)生的時(shí)間或某事多長時(shí)間沒有發(fā)生：She hadn't eaten since breakfast（她早飯后就一直沒吃過東西），It was a long time since she had been to church（她已經(jīng)有很長時(shí)間沒去教堂了）。