Here are the sentences filled in with the appropriate comparative forms:

9. My mother is **younger** than my father.
10. Ha Noi is **smaller** than Ho Chi Minh City.
11. This boy is **taller** than that boy.
12. A dictionary is **thicker** than a notebook.
13. The Great Wall is the **longest** structure in the world.
14. Buildings are **higher** than houses.
15. Lan is the **happiest** student in my class.
16. Our parents are the **best** people.
17. This girl is the **thinner** than in her class.
18. The winter is the **coldest** season in a year.

Feel free to ask if you need more assistance!