过去完成进行时（had been doing)的被动语态是什么?过去完成进行时和过去完成时有什么区别?请举例说明

被动---had been being done
过去完成时：过去某段时间完成的一个动作,一般用had +过去式
以过去某个时间为标准,在此以前发生的动作或行为,
或在过去某动作之前完成的行为,即“过去的过去
如：
As soon as we got to the station, the train had left
当我们到达车站是,火车已经开走了
（我们到达车站时那个动作都已经是过去了,火车开走的时候在我们到达的时候就已经发生了,所以要用过去的过去,即过去完成时）
过去完成进行时： 过去一段时间内持续进行的一个动作,had+been+doing
过去一段时间内一直在做的事情,现在已经终止了
如：
I had been learning English three years ago.
三年前我一直在学英语(但现在没学了);

被动语态
一、各种时态的被动语态举例
一般地讲，被动语态可用于英语的各种时态。为了能准确地运用被动语态，重点是要掌握be动词的各种时态变化。各种时态的被动语态举例如下：
1、一般现在时的被动语态. am / is / are + 动词的过去分词
Our classroom is cleaned every day.
Thi...

1 had been being done
2 He had finished his homework when his mother came back. 妈妈回家时已经做完作业了
He had been finishing his homework when his mother came back. 还没做完，一直在做