在英语语法中，过去完成时是一种重要的时态，用于描述在过去某个时间点之前已经完成的动作或状态。这种时态通常由“had+过去分词”构成，帮助我们更精确地表达事件的时间顺序。

过去完成时的基本结构

- 肯定句：主语 + had + 过去分词 + 其他。

- 否定句：主语 + had not (hadn't) + 过去分词 + 其他。

- 疑问句：Had + 主语 + 过去分词 + 其他？

例如：

- 肯定句：She had finished her homework before dinner.

- 否定句：He hadn't seen the movie before last night.

- 疑问句：Had they arrived when we got there?

过去完成时的应用场景

1. 表示在过去某一时间之前已完成的动作

例如：When I arrived at the station, the train had already left.

2. 与过去进行时结合使用

例如：While she was waiting for the bus, it had started raining heavily.

3. 在复合句中明确动作的先后顺序

例如：By the time he called me, I had already gone to bed.

练习题100道（部分示例）

1. By the end of last year, I ________ (work) here for five years.

2. She ________ (not see) him since they met in 2015.

3. When he opened the door, the children ________ (go) to bed.

4. They ________ (visit) Paris three times before they moved there.

5. By the time I got home, my sister ________ (finish) her homework.

这些练习题旨在帮助学习者更好地掌握过去完成时的运用。通过反复练习，可以加深对这一时态的理解和记忆。

希望这篇文章能帮助你更好地理解和掌握过去完成时及其应用方法。继续努力练习，相信你的英语水平会不断提升！