英语中的完成过去式，即过去完成时，用来描述在过去某一时间点之前已经完成的动作。它的构成是 had + 过去分词。例如：
1. She had finished her homework before she went out to play.
2. By the time they arrived at the station, the train had already left.
3. They had already eaten breakfast when I got up.
这种时态通常用于强调动作的完成或者强调过去发生的事情对现在造成的影响。例如：
I had studied English for 5 years before I moved to the United States. （他搬到美国之前已经学了五年英语。）
Tina had already left when I got there. （当我到的时候Tina已经离开了。）
希望这些例子能帮助你理解过去完成时的用法。