Change the voice...
(Marked with red)

Dear Student

12. A bridge was being built by someone.
13. It is said by them that honesty is the best policy.
15. We were alarmed by the news.
19. He should be helped.
23. The situation must be faced by him.
25. The room must be left at once.
26. The weak must not be insulted.
27. They may have been detained by the rain.
29. A ticket was issued to me by the booking clerk.
30. Everyone was pleased by her melodious song.

Regards.