Past Perfect Continuous Tense.
The Past Perfect Continuous Tense denotes that something has done continually (often, frequently, etc.) in the past or in a (certain or uncertain) past period (of time).
A Few Models.
I had been teaching English on the Web.
He/She had been learning English on the Web.
We had been creating an English website.
When we want to express/stress the period [(of time) (of the action)], we use prepositions “for”, “since”, before, etc. and a (certain or uncertain) past time / period.
Now then, the same Models become:
I had been teaching English on the Web for years. …
He/She had been learning English on the Web since 2012. …
We had been creating an English website before 2016. …
Homework.
Find some more illustrations and make/line/remember the Syntax (as in lessons nos. 76 and 77).