Ⅰ. ago adv. “……以前”指从此刻起，若干时间以前，通常与过去连用。如：
① It happened two days ago.这件事发生在两天以前。
② I met him a few minutes ago.我在几分钟以前碰到他。
Ⅱ. before adv, prep & conj “……以前”指从那时起若干时间以前。通常与完成时、过去时等连用。还可用作前置词或连接词表时间，而ago 则不能这样用。
① He said that he had seen her two days before.他说他两天前见到过她。(表从她说话那时起两天前)
② I had been fine the day before.(那天)前一天的天气很好。
③ I’ve seen that film before.
④ I never met him before.