1. Keeping the environment clean is very important
=> It's important to keep the environment clean
2. They will show the time machine to the public when they finish it.
=>1. The time machine will be shown to the public when they finish it.
3. He was delighted to receive his aunt’s letter.
=>He was delighted that he received his aunt's letter.
4. Old car tires are recycled to make shoes and sandals.
=>People recycle old car tires to make shoes and sandals.
5. I’m very sad that I wasn’t accepted in that group.
=>I'm very sad not to be accepted in that group.
6. It’s three years since I last spoke to her.
=>I haven’t spoken to her for two years.
7. Tam doesn’t type as fast as she used to.
=>tam used to type faster than she does
8. “ Please turn down the radio for me”, said my father.
=>My father asked me to turn down the radio for him
9. Lan will be fifteen next week.
=>It is Lan's fifteenth brithday next week
10. How long is it since you used the pen?
=>When did you use the pen?