過去完成式和過去完成進行式的差別?

The past perfect:-When two actions happened in the past one after another, we use the past perfect tense for the first action:-
eg:-After Lorna had taken a naked bath, she went to the Party !
For unfinished, lasting actions, the past perfect continuous is used together with adverbials beginning with "for" or "since":-
eg:-Linda had been living in London for (2015-3=2012)three years in 2015.
eg:-Linda had been trading in London since 2012.(since=adverbials)
If factors of completion(=unfinished, lasting actions, incompletion) on time are involved, then we use past perfect continuous.
If factors of completion on time are not involved,then we never use the Past Perfect.

1. I had been studying English for three years before I came to this school.
來本校之前，我已學過三年英語了，在到本校的那個時候，仍在學習中。（I was still learning English then.)
用「過去完成進行式」表示那時候已到所述的結果，且未停止，study與come之間是連續的：即「過去完成進行式」的行為雖發生在「過去簡單式」之前，但兩者間有部分是重疊的。
2. I had studied English for three years before I came to this school.
來本校之前，我曾經學過三年英語。
用「過去完成式」時，表示study與come之間可能不連續，而有間隔的：「過去完成式」發生在「過去簡單式」之前。