Definition:     The Perfect Progressive tenses are used to express the time that has elapsed between two points in time.  The Past Perfect Progressive expresses the time elapsed between the beginning of a continuous action that began in the past and another point in time in the past.

Example:     When you called me yesterday at 9:00(point in time), I had been watching the movie for 30 minutes.  I started watching the movie at 8:30 last night and watched it continuously until you called me at 9:00.  In this case we have to use the following sentence:

               When you called me yesterday at 9:00, I had been watching the movie for 30 minutes.

How to write the Past Perfect Progressive:

To write the Past Perfect Progressive you must use the past of the verb To Have, the past participle of the verb To Be, and the –ing form of the verb you are conjugating.