Past Perfect Continuous
had been doing
Действие длилось до момента в прошлом — и важно, сколько оно длилось:
- She had been waiting for an hour when he arrived.
- They had been working on it since morning.
- Причина видимого следствия: His hands were dirty — he had been repairing the car.
Разница с Past Perfect: had done — факт, had been doing — процесс и его длительность.