When do you use do and does?

"Do" is the verb you would use if you are talking about either I or You; otherwise, this word is used with plural subjects. You would say "I do" and "You do" but would say "They do," "Brothers do," "Dogs do" and "Many do." You would use "does" if your subject is singular. "It does," "She does," "My brother does," or "A dog does."