Sure! Here are the sentences you requested based on the provided prompts:

1. It’s three years since I last saw Nam.
- I haven’t seen Nam for three years.

2. The last time she visited me was five months ago.
- She hasn’t visited me for five months.

3. I last wrote to my uncle in July.
- I haven’t written to my uncle since July.

4. Minh began to collect stamps in 2000.
- Minh has collected stamps since 2000.

5. It’s three years since I last spoke to her.
- I haven’t spoken to her for three years.

6. Tam doesn’t type as fast as she used to.
- Tam used to type faster.

7. "Please turn down the radio for me," said my father.
- My father asked me to turn down the radio for him.

8. Nhung will be fifteen next week.
- It’s Nhung’s fifteenth birthday next week.

9. How long is it since you used the car?
- When did you last use the car?

10. Keeping the environment clean is very important.
- It’s very important to keep the environment clean.

11. They will show the time machine to the public when they finish it.
- The time machine will be shown to the public when they finish it.

Let me know if you need anything else!