过去完成式：以过去发生的某件事或时间为参考点，强调之前更早的过去事件已完成（completed by a past time）
Past1 Past2 Present
——————|——————|——————|————→
homework dinner Speech time
Reference
I had done my homework before dinner.
I had done my homework before I ate dinner.
I had done my homework before eating dinner.
——劉美君. (2012). 英文文法有道理! 重新認識英文文法觀念. 聯經.
