Chuyển các câu sau sang dạng bị động

Passive from
12. She didn't send me a letter.
13. We bought this villa two years ago and some builders decorated it last
14. I can't understand the lesson today because it is very difficult.
15. The postman cleans this box three times a day.
16. We serve meals until 23:00. 
17 you don't repair this radio