Introduction:

Good day OP, nice day we're having here. I'll try to help you by answering those questions.

Answer:

1. Luffy got his teeth checked up at the Dentist's.

2. Luffy got his teeth problems by not brushing up his teeth after eating. The fact that he often consumes sweets -particularly chocolate- doesn't help either.

3. As implied in the text, the ideal time to brush your teeth is after having a meal.

4. Luffy vows to his friend that he will start brushing his teeth after every meal and consume much less sweets than what he is used to.

5. Luffy congratulated Daniel for winning a race. As to what kind of race that is, it is not exclusively stated nor implied in the text.

Post Scriptum:

Good luck OP! And please mind your manners as the threat in your question might demotivate someone from responding to you.

Either way, have a good day!