Hello, fellas. In this session we will learn the past perfect progressive.
To begin with, we need to understand the past perfect first. The past perfect conveys an idea that an activity was completed before another activity or time in the past.
(More on the past perfect: https://englishtips4u.com/2011/12/26/engclass-past-perfect-tense/)
Felix had finished his study when Mary started her tertiary education.
First: Felix finished his study.
Second: Mary started her tertiary education.
In the past perfect progressive, the emphasis is put on the duration of an activity that was in progress before another activity or time in the past.
She had been reading a book for two hours before I came.
The past perfect progressive may also express a meaning that an activity was in progress close in time to another activity or time in the past.
He was tired because he had been running.
Betty Schrampfer Azar, Understanding and Using English Grammar: Third Edition