PAST PERFECT CONTINUOUS (HAD + BEEN + -ING) wird normalerweise verwendet, wenn

• die Handlung weiterhin andauert. Im Deutschen steht in diesem Fall im Allgemeinen Präteritum + schon:

       He had been reading many books for the last year.

         They had been living in this flat ever since they moved to London.

• die Handlung wiederholt vor sich gegangen sein kann:

         How long had you been learning Spanish when you were young?

         They had been waiting for half an hour when their friends arrived.

• die Handlung kurz vor einem bestimmten Zeitpunkt in der Vergangenheit beendet worden ist:

        George had just been reading an interesting book when his father called.