Here are the completed sentences using the comparative form of the adjectives and "than":
1. I'm **fatter** than my brother. (short)
*Explanation:* The adjective "fat" becomes "fatter" in the comparative form.
2. This jacket is **cheaper** than that jacket. (cheap)
*Explanation:* The adjective "cheap" becomes "cheaper" in the comparative form.
3. Dogs are **friendlier** than lions. (friendly)
*Explanation:* The adjective "friendly" becomes "friendlier" in the comparative form.
4. Windsurfing is **more exciting** than golf. (exciting)
*Explanation:* The adjective "exciting" becomes "more exciting" in the comparative form.
5. Cosmo is **older** than Suzie. (old)
*Explanation:* The adjective "old" becomes "older" in the comparative form.
6. This car is **more expensive** than that car. (expensive)
*Explanation:* The adjective "expensive" becomes "more expensive" in the comparative form.
7. My bike is **better** than yours. (good)
*Explanation:* The adjective "good" becomes "better" in the comparative form (irregular).
8. Your house is **bigger** than ours. (big)
*Explanation:* The adjective "big" becomes "bigger" in the comparative form.
9. English is **easier** than German. (easy)
*Explanation:* The adjective "easy" becomes "easier" in the comparative form.
10. The blue dress is **nicer** than the red one. (nice)
*Explanation:* The adjective "nice" becomes "nicer" in the comparative form.
11. A cheetah is **faster** than a lion. (fast)
*Explanation:* The adjective "fast" becomes "faster" in the comparative form.
12. Your marks are **worse** than mine. (bad)
*Explanation:* The adjective "bad" becomes "worse" in the comparative form (irregular).
13. A snake is **thinner** than a crocodile. (thin)
*Explanation:* The adjective "thin" becomes "thinner" in the comparative form.
14. A gorilla is **stronger** than a monkey. (strong)
*Explanation:* The adjective "strong" becomes "stronger" in the comparative form.