过去完成时定义:一个动作发生在过去某时刻之前
句子中的动词用:had + 动词的过去分词
例:昨天晚上九点之前,我已经离开了办公室。
翻译:I had (already) left the office by/before 9'clock last night.
补充:left是leave 离开 的过去分词
例:昨天下午三点之前,我已经做完了三张卷子。
翻译:By 3'clock yesterday afternoon, I had (already) finished 3 papers.