A.T. Robertson Commentary Hebrews 12:11

"All chastening seemeth for the present to be not joyous but grievous; yet afterward it yieldeth peaceable fruit unto them that have been exercised thereby, [even the fruit] of righteousness." — Hebrews 12:11 (ASV)

For the present (προς το παρον). A classical phrase (Thucydides), προς with the accusative neuter singular articular participle of παρειμ, to be beside.

Not joyous, but grievous (ου χαρασ, αλλα λυπης). Predicate ablative (springing from) or predicate genitive (marked by). Either makes sense, but note predicate ablative in 2 Corinthians 4:7 (κα του θεου κα μη εξ ημων).

Peaceable fruit (καρπον ειρηνικον). Old adjective from ειρηνη (peace), in N.T. only here and Jas 3:17. Peaceable after the chastening is over.

Exercised thereby (δι' αυτης γεγυμνασμενοις). Perfect passive participle (dative case) of γυμναζω, state of completion, picturing the discipline as a gymnasium like 5:14; 1 Timothy 4:17.