Vincent's NT Word Studies
15. Go (upage). Do not wait for him to come to you.Tell him his fault (elegxon). Rev., shew him. The verb means, first, to test, try, search out; therefore, to cross-examine with a view of convincing or refuting; thence to rebuke or chide. The Rev. shew is better than tell, which implies merely naming the fault; whereas the injunction is, go and prove to him how he has erred. Wyc., reprove, with snub as explanation.
Robertson's NT Word Studies
18:15 {If thy brother Sin against thee} (ean hamartesei adelfos sou). Literally, commit a Sin (ingressive aorist subjunctive of hamartanw). 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} (ekerdesas). Aorist active indicative of kerdainw in conclusion of a third-class condition, a sort of timeless aorist, a blessed achievement already made.