la

Esperanto has exactly one article, la, and it never changes shape: no gender, no number, no case, so it is la homo, la homoj and la manon alike. There is no indefinite article at all, so a bare noun does the work of "a": homo on its own is "a person". Learners tend to either drop la where English keeps it or sprinkle it where English does not, and the safe rule is to use it roughly where English uses "the".