past-perfect progressive tense



  edgood  —  Grammar Tips
There are six progressive tenses. Some grammarians refer to the progressive tense as the progressive aspect of a verb. The progressive tense shows an “ongoingness” of the action denoted by the verb.

The progressive tense is formed by using the verb to be as an auxiliary verb and adding the present participle (-ing) of the verb. Thus, the past-perfect progressive tense is formed by writing the past perfect of the verb to be (had been) and adding the ‑ing verb. Thus:
He had been studying this material for a long time before entering school.

© Grammar.com