Past Perfect Continuous - Quá khứ Hoàn thành Tiếp diễnFORM of Past Perfect Continuous
The Past Perfect Continuous has two parts: the past perfect of the verb to be (=had
been) + the present participle (V-ing) of the main verb. See examples below.
S + had + been + V-ing
I had been learning English.
He had been swimming well.
They had been watching TV.
S + had not + been + V-ing
I hadn't been learning English.
He hadn't been swimming well.
They had not been watching TV.
Had + S + been + V-ing
Had you been learning English?
Had he been swimming well?
Had they been watching TV?

The Past Perfect refers to an action happened in the past and continued until
another action happened. But the speaker is more interested in the process of the
action. USEs of Past Perfect Continuous
1. to describe an action started in the past and continued up until another
time or action in the past.
 He had been playing game for over an hour before Tony arrived.
 She had been applying for that company for three years when it went out
of business.
 How long had you been waiting to get on the train?