Here are the sentences with the correct comparative forms of the adverbs filled in:

1. He arrived **earlier** than I expected. (early)
2. We walked **more slowly** than the rest of the people. (slowly)
3. They called us **later** in the afternoon. (late)
4. My mother and my sister talked **more loudly** than the other guests. (loudly)
5. He hit his arm **harder** than before. (hard)
6. The Spanish athlete ran **faster** than the other runners. (fast)
7. Jim threw the ball **farther** than Peter. (far)
8. We answered all the questions **better** than the other students. (well)
9. Our new teacher explains the exercises **more badly** than our old teacher. (badly) *(Note: "more badly" is correct but it's often better to use the adjective form, "worse," in this context: "worse" instead of "more badly.")*
10. The new mechanic checked the car **more thoroughly** than the old mechanic. (thoroughly)