Here are the mistakes and their corrections:

8. **Mistake:** "visited" (correct form).
- **Correction:** None needed. This sentence is correct as it stands.

9. **Mistake:** "cook" (should be "cooked" for past tense).
- **Correction:** They **cooked** dinner.

10. **Mistake:** "eats" (should be "ate" for past tense).
- **Correction:** Alice **ate** fish.

So the corrected sentences are:
8. Henry visited his parents last week.
9. They cooked dinner.
10. Alice ate fish.