Ответ:

1. When I opened the door, the boy had disappeared.

2. She bought some new glasses after she had gone to the optician's.

3. We had already paid for the clothes before we collected them from the shop.

4. We were late for college because we had missed our bus.

5. Before I read this article, I had never heard of bitcoins.

6. They had kept the receipt when they bought their coputer.

7. After we had visited our cousins, we played football in the park.

8. The realised they had never watched the film before.