1. "They are doing an experiment"
⇒ He said that they were doing an experiment
2. "You have to sign the paper again"
⇒ She told me to sign the paper again
3. Acid rain is dangerous. Trees' leaves are damaged( because of)
⇒ Because of the acid rain, trees' leaves are damaged
4. They built this house yesterday
⇒ This house was built yesterday