Past Perfect Continuous Tense

a. Form: S + Had + Been + Ving
b. Uses

• To express an action was happening before another action in the past. Ex: by 9 o'clock last night, the pilot had been flying nine hours non-stop. They had been waiting for someone when they saw John go with a strange man. The football match had to be stopped. They had been playing half an hour when there was a heavy rain.
• We this tense for an action that went on over a period before a past time. Ex: when I found the file, I'd been looking for it for some time. (The action of looking went on for some time before the discovery of the life.) When I saw Tom, he'd been playing golf. (I saw him after the game.)