在英语语法中，过去完成时是一种重要的时态，用于描述在过去某一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。为了帮助大家更好地掌握这一时态，我们特别准备了以下专项练习。

练习一：选择正确的答案

1. When I arrived at the station, the train ______ already left.

- A) has

- B) had

- C) was

- D) is

2. By the time she finished her homework, it ______ dark outside.

- A) became

- B) had become

- C) becomes

- D) has become

3. They ______ the movie by the time we got to the cinema.

- A) had watched

- B) were watching

- C) watched

- D) have watched

练习二：填空题

4. She ______ (study) for three hours before she went to bed.

5. By the end of last year, they ______ (complete) all their projects.

6. He ______ (forget) his keys when he left home.

练习三：改写句子

7. She realized that she had lost her bag.

改为：She realized that her bag ______ ______.

8. They found that the plane had taken off.

改为：They found that the plane ______ ______.

9. He knew that she had finished her work.

改为：He knew that her work ______ ______.

通过以上练习，希望大家能够更加熟练地运用过去完成时来表达复杂的时态关系。记住，这种时态的重点在于强调一个动作发生在另一个过去动作之前。继续多加练习，相信你的英语水平会有所提高！