一、单项选择题
1. By the time he arrived at the station, the train ________.
A. had left B. has left C. was leaving D. would leave
正确答案：A
解析：由“by the time”引导的时间状语从句表示在过去某一时间之前已经完成的动作，主句应该使用过去完成时。
2. She ________ her homework when I called her last night.
A. did B. was doing C. had done D. has done
正确答案：B
解析：“when”引导的是一个特定的时间点，在这个时间点上她正在做作业，因此应该使用过去进行时。
3. When I got to the cinema, the movie ________ for five minutes.
A. had begun B. had been on C. began D. was on
正确答案：B
解析：电影已经开始并且持续了一段时间，所以这里要用过去完成时的“had been on”。
4. He ________ his wallet before he went out shopping.
A. checked B. had checked C. was checking D. checks
正确答案：B
解析：在出去购物之前检查钱包，这是一个发生在过去某个时间之前的动作，因此需要使用过去完成时。
5. They ________ the meal by the time we arrived.
A. have finished B. had finished C. were finishing D. finished
正确答案：B
解析：我们到达的时候，他们已经完成了用餐，这是一个发生在过去某个时间之前的动作，所以使用过去完成时。
二、改写句子
6. The teacher explained the rule. We understood it immediately.
The teacher ________ the rule before we ________ it.
答案：had explained; understood
解析：根据题意，“理解规则”这个动作发生在“解释规则”之后，但都是过去的动作，因此第一个空格需要用过去完成时。
7. She bought a new dress. She wore it to the party.
She ________ a new dress before she ________ to the party.
答案：had bought; went
解析：买裙子的动作发生在去参加派对之前，所以第一个空格需要用过去完成时。
8. He graduated from college. He started working.
He ________ from college before he ________ working.
答案：had graduated; started
解析：毕业这个动作发生在开始工作之前，因此第一个空格需要用过去完成时。
三、综合运用
9. By the end of last year, they ________ (complete) all the projects.
答案：had completed
解析：到去年年底为止，他们已经完成了所有的项目，这是一个发生在过去某个时间之前的动作，所以使用过去完成时。
10. When I saw him yesterday, he ________ (study) for his exam for two hours.
答案：had been studying
解析：当我昨天见到他的时候，他已经为考试学习了两个小时，这是一个持续的动作，所以使用过去完成进行时。
