USE 1 Completed Action Before Something in the Past
The Past Perfect expresses the idea that something occurred before another action in the past.
I had never seen such a beautiful beach before I went to Kauai.
I did not have any money because I had lost my wallet.
USE 2 Duration Before Something in the Past (Non-Continuous Verbs)
With Non-Continuous Verbs and some non-continuous uses of Mixed Verbs, we use the Past Perfect to show that
something started in the past and continued up until another action in the past.
We had had that car for ten years before it broke down.
By the time Alex finished his studies, he had been in London for over eight years.
Unlike with the Present Perfect, it is possible to use specific time words or phrases with the Past Perfect.
Although this is possible, it is usually not necessary.