1. When I arrived at the station, the train _____ already.
A. left
B. has left
C. had left
D. leaves
答案：C
解析：句子中“arrived”是过去时，而“left”的动作发生在“arrived”之前，所以使用过去完成时。
2. By the time he came back, we _____ the work for hours.
A. had done
B. have done
C. did
D. are doing
答案：A
解析：by the time引导的时间状语从句中，如果主句是过去时，从句中的动作发生在主句动作之前，则从句用过去完成时。
3. She _____ her homework before her mother came back last night.
A. has finished
B. finished
C. had finished
D. will finish
答案：C
解析：last night表明是过去时间，且“finished”发生在“came back”之前，故用过去完成时。
4. They _____ the bridge by the end of last year.
A. have built
B. built
C. had built
D. were building
答案：C
解析：by the end of last year表明是过去的某一时间点之前完成的动作，故用过去完成时。
5. He _____ the book for three weeks before he returned it to the library.
A. has kept
B. kept
C. had kept
D. keeps
答案：C
解析：for three weeks是表示一段时间的持续，且该动作发生在“returned”之前，故用过去完成时。
6-20 题目及解析(简略版)
6. By the time she got to the airport, her flight _____.
A. had left
答案：A
解析：动作发生在到达机场之前。
7. We _____ our dinner when the phone rang.
A. had just finished
答案：A
解析：电话响时，饭刚吃完。
8. Before he left, he _____ a note on the table.
A. had left
答案：A
解析：离开前留下的纸条。
9. She _____ her homework before she went to bed last night.
A. had done
答案：A
解析：睡觉前已完成作业。
10. When I got to the office, the meeting _____ for half an hour.
A. had been on
答案：A
解析：到达时会议已开始半小时。
当然，以下是11-20题带有题干的版本及其答案解析：
11.By the time she got to the park, the concert _____ started.
A. had already
B. has already
C. already had
D. was already
答案：A
解析：句子中使用了“By the time she got to the park”，表示到达公园的时候，音乐会已经开始了。这里需要使用过去完成时来表示“音乐会开始”这一动作发生在“到达公园”之前。正确形式应为“had already started”，但选项中未直接给出完整的动词形式，但从语法结构和语境上判断，A选项“had already”后应接动词的过去分词形式，故A为正确答案。
12.He _____ his homework when his mother came in.
A. had just done
B. had just finished
C. just had finished
D. just finished
答案：B
解析：句子描述的是他妈妈进来时，他刚刚完成作业。由于“came in”是过去时，且“完成作业”发生在“进来”之前，所以应使用过去完成时。同时，“just”用于强调动作的刚刚完成，常与“had finished”搭配使用。
13.I _____ the book for two weeks before I decided to return it.
A. kept
B. have kept
C. had kept
D. am keeping
答案：C
解析：句子中提到“在我决定归还之前”，说明“保留书籍”这一动作发生在“决定”之前，且持续了两周时间。因此，应使用过去完成时来表示这一持续性的过去动作。
14.By the end of last month, they _____ 10,000 trees.
A. had planted
B. planted
C. have planted
D. were planting
答案：A
解析：“By the end of last month”表明是过去的某一时间点之前完成的动作，因此应使用过去完成时。
15.She _____ her luggage before checking in at the hotel.
A. had packed
B. packed
C. has packed
D. was packing
答案：A
解析：“checking in”是过去时，且“打包行李”发生在“入住”之前，所以应使用过去完成时。
16.They _____ the game before the rain started.
A. finish
B. finished
C. had finished
D. will finish
答案：C
解析：“the rain started”是过去时，且“完成游戏”发生在“下雨”之前，故用过去完成时。
17.When we arrived, the concert _____ for ten minutes.
A. had been on
B. has been on
C. was on
D. had been
答案：A
解析：“arrived”是过去时，且“音乐会进行”是一个持续性的动作，在“到达”之前已经持续了十分钟，所以应使用过去完成时的进行时态。
18.She _____ her speech before the audience arrived.
A. had prepared
B. prepared
C. has prepared
D. is preparing
答案：A
解析：“the audience arrived”是过去时，且“准备演讲”发生在“观众到达”之前，所以应使用过去完成时。
19.By the time I got to the station, the train _____ left.
A. has already
B. already had
C. had already
D. was already
答案：C
解析：“By the time I got to the station”表明是过去的某一时间点之前完成的动作，且“left”是动词，需要与助动词“had”搭配形成过去完成时。同时，“already”用于强调动作的完成。
20. He _____ his homework before he went out to play football.
A. had finished
B. finished
C. has finished
D. will finish
答案：A
解析：“went out”是过去时，且“完成作业”发生在“出去玩”之前，所以应使用过去完成时。
