The PAST PERFECT PROGRESSIVE TENSE indicates a continuous action that was completed at some point in the past. This tense is formed with the modal "HAD" plus "BEEN," plus the present participle of the verb (with an -ing ending): "I had been working in the garden all morning. George had been painting his house for weeks, but he finally gave up."
Generally, progressive forms occur only with what are called dynamic verbs and not with stative verbs. If you wish to review that concept now, click