完成的过去式 (Past Perfect) 是指在过去的某一时间点之前已经完成的动作。构成方式为 had + 过去分词。
例如：
- He had studied English for 5 years before he moved to the United States. (他搬到美国之前已经学了五年英语。)
- By the time she arrived at the station, the train had already left. (她到达车站时，火车已经开走了。)
完成的过去式在英语中也常被用于虚拟条件句 (Conditional Sentences) 中的第三种情况，即表示过去的虚拟条件。构成方式为 If + 主语 + had + 过去分词，主句则使用 would/could/should have + 过去分词。
例如：
- If I had known the answer, I would have told you. (如果我知道答案，我就会告诉你。)
- If we had left earlier, we could have caught the last train. (如果我们早点出发，我们还能赶上最后一班火车。)
