Решение:
- 11. Were the shops open last Saturday?
- 12. Were we in the mountains two months ago?
- 13. Was Dave in Moscow three weeks ago?
- 14. Were the books on the piano two hours ago?
- 15. Was he a good pupil many years ago?
- 16. Were they in the kitchen a minute ago?
Обоснование: Для образования вопросительного предложения глагол 'to be' (was/were) ставится перед подлежащим.