This verb tense describes a past, ongoing action that occurred before another past action in time.

The past perfect progressive tense is generally used in a sentence to depict an action that started at some time in the past and continued until a specific time in the past. It refers to an action that had been progressing until a certain point in the past.

For example:

• Before her family moved, she had been attending a private school.
• I had been waiting to hear from the admissions officer for more than a month before I applied to other colleges.