Sure, here are the correct forms of the adjectives to fill in the blanks:

1. I rarely wear my brown shoes because they are **less comfortable** than the black ones.
2. This pair of trousers is not big enough. Do you have a **bigger** one?
3. This bag costs too much. Do you have a **less expensive** one?
4. My mother is **less busy** on Saturday than other days. She doesn't have much to do on that day.
5. I enjoy being with my father. He is **more easygoing** than my mother.
6. Going shopping in a mall is **more convenient** than shopping in a thrift store. There are **more** choices.
7. This dress looks **prettier** than others. It is the **best** one in the shop.
8. I rarely eat noodles. They are my **least favourite** food.
9. Helen learns things very quickly. She is **cleverer** than her classmate Mary, and she always gets **better** grades than Mary. Mary is **less intelligent** than Helen.
10. I live in a quiet street. There are not many people here. This street is **less crowded** than others in the town.
11. Life in the countryside is **less exciting** than life in the city. It's **slower** and **more boring**.
12. Most students can answer Question 1 because it is **less difficult** in the test.
13. Not any students in my class can answer Question 1 because it is **the most difficult** in the test.