1.It often last winter. 2.They Russian last year. 3.Mike

1. It often snowed last winter. 2. They studied Russian last year. 3. Mike invited his friend to dinner. 4. Many of our pupils skated and skied last Sunday. 5. The boy missed some geography and chemistry lessons last month. 6. I wanted to wait for you but I couldnt. 7. My mother kept books on the shelf. 8. They found a lot of books there. 9. There were beautiful flowers in the garden. 

В предложениях 7 и 9 время не обозначено, его нельзя определить из контекста, потому можно также использовать глагольные формы the Present Simple: 7. My mother keeps books on the shelf. 9. There are beautiful flowers in the garden.