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(fā)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 I’ve seen that film before.
④ I never met him before.
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