Here are the mistakes identified in each sentence:

16. **Mistake:** "meal" should be plural.
**Correction:** "You should have several small **meals** a day."

17. **Mistake:** "keep away from" should be "keep" or "adopt" healthy habits and activities.
**Correction:** "You should **adopt** healthy habits and activities."

18. **Mistake:** "most" should be "more."
**Correction:** "The **more** you study, the smarter you will become."

19. **Mistake:** "eats" should be "eat."
**Correction:** "People who skip breakfast **eat** more at the next meal."

20. **Mistake:** "larger" should be "largest" to indicate the superlative form.
**Correction:** "It is important to remember that dinner doesn't have to be the **largest** meal."

Feel free to ask if you need further explanations or corrections!