In Hindi, transitive and intransitive verbs have different rules in the past tense. Intransitive verbs the past conjugation is enough, but for transitive verbs like "to eat" there needs to be a particle to explain who did the eating, so ने is added as a postposition to indicate which subject did the eating.
आपने संतरे खाये
लड़के ने संतरे खाये
ने is needed with these verbs to show who did them in the past tense.
(Not a native speaker so take my advice with a grain of salt)