I (or You，He，John，They，Those men) had been studying.
过去完成进行时与过去完成时的用法很相近，它通常表示一个持续到过去某时的动作：
He had been waiting for two weeks. He was still waiting.(有一个表示一段时间的状语)
Up to that time he had been translating those books.(He was still translating at “that time”.)
He had been writing the letter till two o'clock.
He was now thirty years old.He had been gambling since he was ten.(He was still gambling at thirty.)
He had been standing there in the sun.(没有时间状语)
He had been thinking about his marriage.
不过这个时态并不一定表示这个过去动作将持续下去：
He came back at seven.He had been waiting for her two hours.(At seven he did not wait any more.)
He stopped swimming.He had been swimming for the last three hours.
另一方面，这个时态也和现在完成进行时一样，也可有一些特别的含义：
1)尚未完成：
He had been writing the novel.(He had not finished it yet.)
2)企图：
He had been studying the meaning of this proverb.(He was trying to study it.)
3)未得结果：
We had been studying what our enemy had said.(But we were not able to understand it.)
4)最近情况：
He had been quarrelling with his wife.(lately)
5)反复动作：
He had been asking me the same question.(Many times)
6)情绪：
What had he been doing?(不耐烦)
这个时态很少用在否定句中，而多以过去完成时代替：
He had not practised English for many years.(普通说法)
He had not been practising English for many years.(少见)
这个时态还可用在said，supposed等引起的间接引语中，代替现在完成进行时：
He said，“I have been speaking to John.”= He said that he had been speaking to John.
He thought，“She was watching me when I passed.”= He thought that she had been watching him when he had passed.