过去完成时表示过去某个时间以前已经完成的动作，强调“过去的过去”。常以before, by+过去时间，或when, before等引导的过去时态的从句来表示，也可以通过上下文来表示。
【例如】
When I arrived at the station, he had already left.We had learned about 4000 English words by the end of last term.I waited until he had finished his homework.We were surprised at what she had done.We____our breakfast when an old man came to the door. (CET-4 1990,1)
A) just have had
B) have just had
C) just had
D) had just had
本题吃饭的动作在来的动作之前，考察过去完成时的用法,正确答案为D.
Before the first nonstop flight made in1949, it____necessary for all planes to land for refueling. (CET-4 1996, 6)
A) would be
B) has been
C) had been
D) would have been
本题时间状语从句为一般过去时，主句的情况是在从句之前，是“过去的过去”，因此用过去完成时，答案为C.
Until then, his family____from him for six month.( CET-4 1997, 1)
A) didn't hear
B) hasn't been hearing
C) hasn't heard
D) hadn't heard
过去完成时可以表示过去某一时间以前已经发生并继续发生的事情，本句考察的正是过去完成时的这一用法，故答案为D.
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