"She does not eat in the afternoon."
afternoon is not plural in english, when used in the general sense. does it always have to be plural in portuguese?
Thats not so common. We usually say "à tarde" or informally "de tarde"
Although the word afternoon is not plural the sentence means that she does not eat on any afternoon, of which there are many. I guess in Portuguese the fact there are many afternoons translates as plural. I think the same thing can be seen in translations regarding days of the week and time also.