1. It's very important to keep the environement clean.

2. The time machine will be shown to the public when they finish it.

3. He was delighted that he received his aunt's letter.

4. People recycle old car tires to make shoes and sandals.

5. I'm very sad not to be accepted in that group.

6. I haven't spoken to her for three years.

7. Tam used to type fast.

8. My father asked me to turn down the radio for him.

9. It is Lan's fifteen birthday next week.

10. When did you last use the pen ?

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1. It's very important to keep the environment clean.
2. The time machine will be shown to the public when it is finished.
3. He was delighted that he received his aunt's letter./ He was dleighted his aunt sent him a letter.
4. People recycle old car tires to make shoes and sandals.
5. I'm very sad not to be accepted in that group.
6. I haven't spoken to her for three years.
7. Tam used to type faster than she does now.
8. My father asked me to turn down the radio for him.
9. It is Lan's fifteenth birthday next week. 
10. When did you use the pen?