It depends on wich pronoun! You can't translate it without the pronoun, the conjugations change for all the person, unlike English where several persons share the same conjugations.
Eu comi =I ate./ Você comeu =You ate. /Ele (Ela) comeu=He ate/ Nós comemos. = We ate / Vocês comeram =You ate. / Eles (Elas) comeram =They ate.
The subject and the complement of the action are not related. When you say: "The boy buy a female dog", there's no link between the gender of the boy and the gender of the dog. Uma is feminine because maçã is feminine. Ele is masculine because the person who eat is masculine, no links.