A.T. Robertson Commentary Matthew 18:15

"And if thy brother sin against thee, go, show him his fault between thee and him alone: if he hear thee, thou hast gained thy brother." — Matthew 18:15 (ASV)

If thy brother sin against thee (εαν αμαρτηση αδελφος σου). Literally, commit a sin (ingressive aorist subjunctive of αμαρτανω). Aleph B Sahidic do not have "against thee" (εις σε).

Shew him his fault (ελεγξον). Such private reproof is hard to do, but it is the way of Christ.

Thou hast gained (εκερδησας). Aorist active indicative of κερδαινω in conclusion of a third-class condition, a sort of timeless aorist, a blessed achievement already made.