Tiempos verbales: Past perfect continuous

Cuándo se usa:
- cuando una acción que se estaba realizando en el pasado antes de otra ya acabada.
- para mostrar el resultado de una acción pasada.
- con expresiones: before, when, for, since, by the time and how long.

Estructura:
Sujeto + had + been + verb -ing + ....
Sujeto + hadn't + been + verb -ing + ...
Had + sujeto + been + verb - ing + .. + ?

Ejemplos:

I had been cooking all the morning, so I needed a break
She had been waiting for 15 minutes when they arrived
You had been working hard since the previous month.