過去完成時是英語中的壹種時態，表示過去某個時間點或另壹個過去動作之前已經發生的動作或狀態。過去完成時的構成是：had + 過去分詞。例如：
- I had finished my homework by the time my parents came home.
- She had already left before I arrived.
過去完成時的使用需要註意以下幾點：
1. 表示過去的過去，即過去的過去。例如：I realized I had left my phone at home after I arrived at the office.
2. 表示某個時間點之前已經完成的動作。例如：By the time I woke up, my roommate had gone to work.
3. 表示過去某段時間內已經完成的動作。例如：She had been studying English for 3 years before she came to England.
除了以上的用法之外，過去完成時還可以用於虛擬語氣的表達，例如：
- If I had known the answer, I would have told you.（如果我知道答案，我就告訴妳了。）
總的來說，過去完成時是英語中較為高級的語態之壹，需要練習和掌握。