I
1. was destroyed
2. are looking
3. Have-found
4. is
7. invented
8. will meet 
9. painted
II.
11. When you have a post – office box, the postman does not bring letters to you, but you go to the office and get your letters and parcels from your box.
12. Because the box is locked, and you have the key, so the letters are quite safe.
13. The headteacher of a school wrote to the post-office because  the headteacher asks for a post-office box for his school