Chuyển các câu sau sang thể 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.
18. Quang usually does his homework at night.
19. I don't like people laughing me at my mistake.
20. Did anyone see you come in