Importante
Past perfect continuous or simple?
i) Remember that some verbs (the stative) are never used in the continuous, e.g. belong, know, own, etc.
ii) To emphasise the activity use the continuous. To talk about things completed, use the simple:
He had been writing letters when I arrived. (we don't say how many- emphasis on activity)
He had written 20 letters when I arrived. He had been writing 20 letters (because it means that he didn't finish any letters)
iii) sometimes they can both be used but simple indicates more permanence:
The castle had stood on the rock for hundreds of years until it was destroyed by the Romans.
The man had been standing there for hours- he was absolutely soaked.