Q：我們不能說 I have met him five years ago.，而必須說 I have met him before.。為什麼 before 可以跟現在完成式連用，而 ago 卻不行呢?

A：before 和 ago 皆意為「以前」，但前者指的是「現在之前的任何時間」(亦即指過去某個不確定的時間)，而後者是指「現在之前的某個確定的時間」。所以，before 可以跟現在完成式連用，而 ago 只能用於簡單過去式。例如：

• I know that guy. I’ve met him somewhere before. (我知道那個人。我以前在某個地方見過他)
• Have you been here before? (你以前有來過這裡嗎?)
  No, I’ve never been here before. This is my first time. (沒有，我以前從未來過這裡。這是第一次。)
• Your mother phoned five minutes ago. Can you phone her back? (你母親 5 分鐘前來電。你可以給她回電嗎?)
• I saw her for the first time at the National Palace Museum some twenty years ago. (我約 20 年前在國立故宮博物院首次見到她)

before 通常跟現在完成式連用，但也可用於簡單過去式。

• I know that guy. I met him somewhere before.
• Were you here before?
  No, I was never here before. This is my first time.
• We were all students before. (我們以前都是學生)

ago 所指的時間都是從現在開始往以前推算，但 before 所指的時間除了上述也是從現在開始往以前推算之外，還可指從過去某個時間點往更早之前推算的時間。所以，當我們要表示比 ago 更早的時間時，我們只能用 before (that) 或 earlier 或 previously，而不能用 ago。在這種句子中，ago 用簡單過去式，而 before 用過去完成式。例如：

• My grandfather died five years ago; my grandmother had already died three years before (that)/previously/earlier. (我祖父 5 年前過世，而我祖母在那之前 3 年就已經過世了 — 亦即祖母是在 8 年前過世)
• I met my ex-girlfriend at a hotel in Bangkok in December 2006 when she told me that she had got married four years before. (2006 年 12 月我在曼谷一家飯店遇見我前女友，當時她告訴我，她在那之前 4 年就已結婚了 — 亦即她在 2002 年結婚)
• Last year I went back to my hometown that I had left ten years before. (去年我回到暌違 10 年的故鄉 — 亦即我在 11 年前離開故鄉)