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.