Student or Learner
Has or have
1 "Everybody has to do their work or have to do their work"
2 "Your hair have become thin or has become thin"
Re: Has or have
In #1, use "has"; it is a singular pronoun.
In #2, use "has"; it is a singular noun.
Don't be confused by "their" in number 1. It was used as a gender-neutral pronoun and intended to be singular. One could use "his/her" there.