Here are the correct forms of the adjectives:

1. Your bag’s zip is easier to close than mine.
2. My mother’s advice is always useful.
3. This summer is drier than ever.
4. It is the most difficult rule of all.
5. This pen writes better than my previous one.
6. This athlete is stronger than his competitor.
7. This pupil is the cleverest in the class.
8. This is the oldest castle in Britain.
9. Health is more important than money.
10. This path is narrower than the parallel one.
11. This actor is the most famous actor in Spain.
12. Kenyans are the best sprinters.
13. Luxurious hotels are more expensive than economic ones.
14. My older sister is afraid of mice.
15. Saturn is the 2nd largest planet in our solar system.
16. We have more homework today than we had yesterday.
17. If you need any further information, please contact our assistant.
18. The second part of the film is more boring than the first.
19. In childhood my aunt was thinner than she is now.
20. This highway is wider than that highway.
21. My house is near to the library.
22. We live in the smallest country in the world.
23. My best friend lives in the country.