Past continuous = was/were + present participle = Karen was walking. It talks about something going on at a specified moment in the past. Past perfect continuous = had been = Karen had been walking. It talks about something that started before a specified moment in the past and was continuing non stop until that moment.

See if two examples can help. 1. I was taking a bath when the telephone rang. So I could not answer it. 2. I had already been learning French on my own for some time when my boss proposed sending me to a French class on corporate expenses, with a view to transferring me to the Paris office in 6 months' time.

Well, I guess you're already very aware of how past continuous and past perfect function differently. Past perfect continuous combines them.