Здесь нужно сверху, над глагольными формами, указать последовательность событий с цифр 1 и 2 (1 - что случилось раньше, 2 - что произошло потом).
2 1
1. By the time I left the building everyone had gone home. (то есть, сначала все ушли, потом я)
2 1
2. I looked in the garden, but they had already gone to work.
1 2
3. They had cleaned everything up by the time their parents returned.
1 2
4. I had already taken three aspirins, but they didn't help.
2 1
5. I wanted to have a bath, but someone had used all the hot water.
1 2
6. We had made our plans carefully and won the prize.
