different usages of this tense:
Typical Time Expressions
Make sure you also read:
(The verbs are
He had been driving for hours before he arrived.
He had been carrying that computer for an hour before he put it down.
He had been fighting
with that machine
for a long time until the mechanic arrived.
They danced so nicely because they had been practicing.
He had been exercising so he could lift it easily.
He fell asleep because he had been working all night.