Ответ:
1. They have postponed the conference. The conference has been postponed by them.
2. Somebody is using the computer at the moment – The computer is being used by somebody at the moment.
3. I didn't realize that somebody was recording our conversation - I didn't realize that our conversation was being recorded by somebody.
4. They have built a new hospital near the airport – a new hospital has been built by them near the airport.