:动词时态和语态重点讲解

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  3） 过去完成时的时间状语before, by, until , when, after, once, as soon
  as。例如：
  He said that he had learned some English before. 他说过他以前学过一些英语。
  By the time he was twelve, Edison had began to make a living by himself. 到了十二岁那年，爱迪生开始自己谋生。
  Tom was disappointed that most of the guests had left when he arrived at the party.
  汤姆失望了，因为他到达晚会时，大部分客人已经走了。
  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。