Раскройте скобки, употребив глагол в соответствующем времени

1. it has been raining since morning.2. When I saw him, his clothes were dirty as he had been working in the garden since morning.3. Where is Nick? He is buying a book now.4. Every day she writes letters and sends cables.5. When I came back home my son was drawing something.6. He will have painted the picture by the end of June.7. We had already discussed the plan when he came. 8. They are students now.9. It's a pity, but I have not finished the work.10. We have just discussed the mistakes in our works.11. He has never been to St. Petersburg.12. My friend will leave for Moscow at 5 o'clock p.m.13. When were you there last?