Here are the completed sentences with the adjectives in the correct comparative and superlative forms:

1. This chair is **more comfortable** than that one. (comfortable)
2. Your flat is **larger** than mine. (large)
3. The weather today is **hotter** than it was yesterday. (hot)
4. The Nile is the **longest** river in the world. (long)
5. Chinese bicycles are **worse** than Japanese ones. (bad)
6. Mathematics is **more difficult** than English. (difficult)
7. Ho Chi Minh is the **biggest** city in Vietnam. (big)
8. He drives **more carefully** than his friend. (careful)
9. She sings **more beautifully** in this school. (beautiful)
10. I read **slower** than my sister. (slow)
11. Grace is the **oldest** girl in our class. (old)
12. This exercise is **easier** than that one. (easy)
13. He is twice as **fat** as you. (fat)
14. Nam is the **noisiest** student of all. (noisy)
15. My cold is **better** today than it was yesterday. (good)
16. This exercise is the **easiest** of all. (easy)
17. This flower is **more beautiful** than that one. (beautiful)
18. He has twice as **many** books as his sister. (many)
19. Which is the **most dangerous** animal in the world? (dangerous)
20. English is not so **difficult** as Chinese. (difficult)