• 過去完成式常用於描述過去發生的兩個動作，先發生的用過去完成式，後發生的用過去簡單式
  例如:
  I had never cooked before I went to college.
  I had already finished my homework when he called.

• 比較現在完成式與過去完成式的句意差異

  1. 現在完成式代表到目前為止一直都持續的狀態
     Mr. Wang has worked for our company for 12 years.
     王先生在我們公司任職十二年。(現在仍然在職)

  2. 過去完成式則表示某個曾經持續了很久的狀態或動作(已在過去的某個時間結束了)
     Mr. Wang had worked for our company for 12 years before he found that job.
     在找到那份工作之前 王先生曾在我們公司任職十二年 (目前已不在本公司任職 )

不規則變化

• 某些動詞的過去過去分詞是不規則的
  例如:
  be動詞的過去分詞為been
  do的過去式為done
  have/has的過去式為had

• 其餘常見不規則動詞請參考Cambridge University Press出版的不規則動詞表。(點我下載PDF)

1. 使用正確過去完成式動詞形 had + P.P.
   例句：
   I has finished my homework when he came. (X)
   I had finished my homework when he came. (O)
   
   This type of thing had never happen before. (X)
   This type of thing had never happened before. (O)

2. 現在完成式或過去完成式?
   例句：
   The rain has already stopped when I left for school. (X)
   The rain had already stopped when I left for school. (O)
   說明: 比某個過去式動作(I left for school)更早發生的事(雨停)，要用過去完成式
   
   Mr. Wang had worked for us for over fifteen years. He’s one of our best employees. (X)
   Mr. Wang has worked for us for over fifteen years. He’s one of our best employees. (仍在職) (O)
   說明: Mr. Wang直到現在仍然是我們的員工，因此截至目前為止人仍繼續為我們工作，該動作並未終止，故使用現在完成式

3. 過去簡單式或過去完成式?
   例句：
   By the time we had got to the station, the train left. We missed the train. (X)
   By the time we got to the station, the train had already left. (火車先出發我們才到達車站) (O)
   說明: 若兩個都發生在過去的動作一前一後發生，先發生的用過去完成式，後發生的用過去簡單式。