If thy brother sin against thee (ean hamartēsēi adelphos sou). Literally, commit a sin (ingressive aorist subjunctive of hamartanō). Aleph B Sahidic do not have “against thee” (eis se).
Shew him his fault (elegxon). Such private reproof is hard to do, but it is the way of Christ.
Thou hast gained (ekerdēsas). Aorist active indicative of kerdainō in conclusion of a third-class condition, a sort of timeless aorist, a blessed achievement already made.