수동태를 능동태로 바꾸는 방법이 궁금해요

시험에 나오는 수동태 문장을 능동태로 바꾸는 방법을 잘 모르겠어요.
어디서부터 어떻게 바꿔야 할지 헷갈립니다.
아래 문장들 설명 부탁드려요.
1. When I heard from my agent that I was asked to sing two songs for your charity, I was really pleased.
2. The paper should remain in the bottom when the glass is turned upside down.
3. If the glass were turned on its side under water, the air would be released and then water would fill the glass.
4. Since all materials that we use are taken from the earth, they have to be returned in order to be used again.
5. Nothing is created or destroyed, but everything is changed in form.
6. An apple is obtained from the earth.
7. A cycle is set up.