John 13:38

Greek "ἕως οὗ ἀρνήσῃ". ἀρνήσῃ "he will deny" is a 3rd person masculine form similar to what is used in Matt. 10:33 ἀρνήσηταί "whoever might deny."  It is properly translated as "until he denies" or "until he renounces."

In Greek, verbs are conjugated according to the person, number, tense, mood, and voice of the subject. The form "ἀρνήσῃ" (arnēsē) is the third person singular aorist subjunctive active of the verb "ἀρνέομαι" (arneomai), which means "to deny" or "to refuse."

"οὗ" (hou): This is a relative pronoun meaning "whom/whomever" "which/whichever".

When combined, "οὗ ἀρνήσῃ" could be translated as "where/when/which/whom he will deny" depending on the context. We know it is not a place, thing, or circumstance that is the object of denial, but someone. Therefore we translate it "whom he will deny." It indicates a future action of denying or refusing, with the relative pronoun specifying the circumstances or time frame in which the denial will occur.