(What do we need to do to maintain having a beautiful teeth?)

We have to brush our teeth three times a day.
>This sentence is correct.

Unless we brush the teeth, we have cavity.
> If we will not brush our teeth, we will have cavities.

 And then it makes us hurt.
And then, it makes our teeth hurt.

Thank you for making your homework, Eric. Keep improving your skills by speaking and writing. Study well and have a nice day ahead. ^^

~Teacher Cassie