Употребление: для обозначения длительного(период времени) действия, начавшегося и закончившегося в прошлом с акцентом на длительность. He had been working on the computer for an hour before she came to help him. Everything had been going well up to then. The driver who died in an accident had been drinking. Второй момент-видимый результат длительного действия в прошлом. My hands were wet.I had been washing the floor. She was tired. She had been working hard all day yesterday. Time markers: for,since,how long,before,etc.