Here are the rewritten sentences using the words in brackets:

5. She is more beautiful than her younger sister. (as)
**She is as beautiful as her younger sister.**

6. They have studied English since 2004. (began)
**They began studying English in 2004.**

7. I don’t have time to collect stamps as when I was in primary school. (used)
**I used to collect stamps when I was in primary school.**

8. Did you often go to the beach when you lived in Nha Trang? (use)
**Did you use to go to the beach when you lived in Nha Trang?**

9. The market does not have any carrots. (There)
**There are no carrots in the market.**

10. Keep the environment clean or we will suffer from pollution.
**If we do not keep the environment clean, we will suffer from pollution.**