A.T. Robertson Commentary 2 Corinthians 12:21

"lest again when I come my God should humble me before you, and I should mourn for many of them that have sinned heretofore, and repented not of the uncleanness and fornication and lasciviousness which they committed." — 2 Corinthians 12:21 (ASV)

When I come again (παλιν ελθοντος μου). Genitive absolute. Paul assumes it as true.

Lest my God humble me (μη ταπεινωση με ο θεος μου). Negative final clause (μη and first aorist active subjunctive), going back to φοβουμα in 20. He means a public humiliation as his fear. The conduct of the church had been a real humiliation whether he refers to a previous visit or not.

That have sinned heretofore (των προημαρτηκοτων). Genitive plural of the articular perfect active participle of προαμαρτανω to emphasize continuance of their sinful state as opposed to μη μετανοησαντων (did not repent) in the aorist tense.