过去完成时是英语中的一种时态，常见的引导词有”had + 过去分词”、”by + 过去时间”、”before”和”when”。这些引导词用于表示在过去某个时间点之前已经完成的动作或事件，以及两个过去的事件之间的先后顺序。
- had + 过去分词
这是最常见的过去完成时引导词，通常放在主语之后，用来表示在过去某个时间点之前已经发生的动作或事件。例如：
- I had finished my homework by the time my mom came home.
- They had already left when we arrived at the party.
- by + 过去时间
这个引导词通常用来表示在过去某个时间点之前的动作或事件已经完成。例如：
- By the time I got to the cinema, the movie had already started.
- She had written three chapters of her book by the end of the week.
- before
这个引导词也是用来表示在过去某个时间点之前的动作或事件已经完成。例如：
- He had finished his presentation before the meeting started.
- They had left the house before we got there.
- when
这个引导词通常用来表示两个过去的事件之间的先后顺序，其中过去完成时表示先发生的事件。例如：
- When I arrived at the airport, the plane had already taken off.
- She had already eaten breakfast when I woke up.
