Affirmative form:

Perfect tenses are formed with the auxiliary verb "have" + past participle of the verb you want to conjugate. Therefore, the past perfect is formed : past of the aux. V. "have" + a verb in past participle:

When John arrived home, his flatmates had already left for the airport.

  Negative form (... aux.V + not + ... ):

Jennifer had not finished her homework when her father said it was dinner time.

  Interrogative form (...aux.V + Subj. + ... ):

Had Margaret finished cooking lunch when her guests arrived?

  an action or situation that happened before a particular time in the past:

	The pilot ejected from the plane because two of the engines had caught fire.
	Catch (an action in the past) is in past perfect because it expresses an action that happened before the one expressed by eject (also in the past).

  an action or situation which started before a particular time in the past and was still in progress at that time, stopped at that time or just before it:

Paul, who had been an exemplary worker for 20 years, was tired of  being called whenever there was a problem.