Ⅰ. ago adv. 以前指從此刻起，若干時間以前，通常與過去連用。如：

　　① It happened two days ago.這件事發生在兩天以前。

　　② I met him a few minutes ago.我在幾分鐘以前碰到他。

　　Ⅱ. before adv, prep conj 以前指從那時起若干時間以前。通常與完成時、過去時等連用。還可用作前置詞或連接詞表時間，而ago 則不能這樣用。

　　① He said that he had seen her two days before.他說他兩天前見到過她。(表從她說話那時起兩天前)

　　② I had been fine the day before.(那天)前一天的天氣很好。

　　③ Ive seen that film before.

　　④ I never met him before.