We often use a clause with since to show when something started in the past:
They had been staying with us since the previous week.
I was sorry when the factory closed. I had worked there since I left school.
I had been watching that programme every week since it started, but I missed the last episode.
when we are reporting our experience and including up to the (then) present:
My eighteenth birthday was the worst day I had ever had.
I was pleased to meet George. I hadn’t met him before, even though I had met his wife several times.
for something that happened in the past but is important at the time of reporting:
I couldn’t get into the house. I had lost my keys.
Teresa wasn’t at home. She had gone shopping.
We use the past perfect to talk about the past in conditions, hypotheses and wishes:
I would have helped him if he had asked.
It was very dangerous. What if you had got lost?
I wish I hadn’t spent so much money last month.