PAST PERFECT CONTINUOUS TENSE

Like the past perfect tense, the past perfect continuous refers to events or actions that ocurred before a particular point in the past. Like other continuous tenses, the past perfect continuous focuses on the duration of activity.
EX: He failed his driver´s test. (past event)
      He had been practicing for a long time (activity that lasted for a long time)

• Affirmative sentence:

     subject  +  HAD + BEEN + VERB+ING  +  _________________
     pronoun
     ex: They had been studying for many years to get this certification

• Negative sentence:

      subject   + HAD  + NOT  + BEEN  +  VERB + ING   + ______________
      pronoun        (Hadn´t)
      ex: she hadn´t been preparing herself for this shocking event

• Question form:

       HAD   +   subject    +   BEEN  +   VERB+ING   +  ____________ ?
                     pronoun
       EX: Had they been playing soccer for such a long time?

• Short answers:

       Yes,  + subject pronoun + HAD           
       No,    + subject pronoun + HADN´T

Do you want to know more about the past perfect continuous tense? go to :
http://www.englishpage.com/verbpage/pastperfectcontinuous.html