11. They know that English is an international language.

English is known to be an international language.

12. Don't do that silly thing again.

Don't let that silly thing happen/ occur again.

13. We'll continue to use this phone.

This phone will continue to be used.

14. Did people make jeans two hundred years ago?

Were jeans made two hundred years ago?

15. They opened the road 10 years ago.

The road was opened 10 years ago.

16. You should open the wine 3 hours before you use it.

The wine should be opened 3 hours before it is used.

17. It's your duty to make tea for the party.

You are supposed to make tea for the party.

18. The man gave me a piece of paper with his address on it.

I was given a piece of paper with the man's address on it.

19. The teacher is watching them work.

They are being watched to work by the teacher.

20. What subjects have people discussed today?

What subjects have been discussed today?