过去完成时表示在过去某一时间或动作之前已经发生或完成了的动作。
eg. I had received 6 letters by last night.截止到昨晚，我收到六封信。
现在完成时指过去发生并且已经完成的动作对现在造成影响或后果，过去某一时间开始并一直持续到现在的动作或状态。
I have lived in this city since 2007.
这个很简单：
过去完成时即过去的过去：我昨天说我已经完成了作业，I said I had finished my job.
将来完成时即将来的过去，但现在还没完成：我后天说我明天就会已经完成了作业，I say I will have finished my job on that day.
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