B1
Past Perfect Continuous (had been + -ing)
Длительность до другого момента в прошлом.
Форма
had been + -ing: She had been waiting for an hour when the bus finally came.
Длительность до события в Past Simple
I was tired because I had been working all day — усталость от процесса до той точки. Вопрос: How long had you been living there before you moved?
vs Past Perfect (had + V3)
Past Perfect — результат или факт: He had finished. Past Perfect Continuous — процесс: He had been studying for hours (акцент на длительности).