Affirmative form:

Continuous tenses are formed with the aux. verb be in the tense required by the situation, in this case past perfect + present participle of the verb you want to conjugate:
 
I had been working for a while when all my friends arrived.

  Negative form:

John had not been working long when he had to stop because of a power cut.

  Interrogative form : 

How long had you been working when your friends arrived? 

USES

  when the action expressed by the verb began before a particular time in the past and continued up to that time, or stopped just before it:

We decided to stop for the night after we had been walking all day.

  sometimes to express a repeated action in the past perfect:

He had tried five times to catch his parrot, which had escaped from its cage  = He had been trying to catch his parrot, which had escaped from its cage.