Ответ:

1. had forgotten

2. had gone

3. had just had

4. had left

5. hadn't phoned

6. had come back

7. had met

8. often had heard of, had never seen

9. had given up, had made

10. had already bought

11. had arranged

12. had sold

13. had finished

Объяснение:

Просто используй глагол had и прибавь к нему смысловой глагол в форме причастия прошедшего времени (в "третьей" форме).