1. People balive that 13 is an unlucky number.
=> 13 is believed to be an unlucky number
2.They know that English is an international language.
=> English is known to be an international language.
3. I have heard her sing this song several times.
=> This song has been heard to sing several times by her
4. People saw him steal your car.
=> He was seen to steal your car
5. They say that John is the brightest student in class.
=> It's said that John is the brightest student in class.
6. Open your book.
=> Your book is opened
7. She asvised me to sell that house.
=> I was asvised to sell that house by her
9. They think that he has died a natural death....
=> It's thought that he has died a natural death.
10. He won't let you do that silly thing again.
=> You won't be let to do that silly thing again
11. He orders us to clean the floor.
=> We are ordered to clean the floor by him
12. You must see it to belive it.
=> It must be seen to belive it
13. People belive that the earth stood still.
=> It's belived that the earth stood still.
14.People son't make the children work hard.
=> The children aren't made to work hard
15. Don't do that silly thing again.
=> That silly thing isn't done again
16. They begged us to help him.
=> We are begged to help him by them