过去完成时典型例题
The students ___ busily when Miss Brown went to get a book she ___ in the office.
A. had written, left　
B. were writing, has left
C. had written, had left　
D. were writing, had left
答案D. "把书忘在办公室"发生在"去取书"这一过去的动作之前，因此"忘了书"这一动作发生在过去的过去，用过去完成时。句中when表示的是时间的一点，表示在"同学们正忙于……"这一背景下，when所引导的动作发生。因此前一句应用过去进行时。

注意：
had no … when　还没等…… 就……
had no sooner… than　刚…… 就……
He had no sooner bought the car than he sold it.