Finding fault with them (μεμφομενος αυτους). Present middle participle of μεμφομα (cf. αμεμπτος), old verb, in N.T. only here and Romans 9:19. The covenant was all right, but the Jews failed to keep it. Hence God made a new one of grace in place of law. Why do marriage covenants so often fail to hold? The author quotes in verses Hebrews 8:8-12; Jeremiah 38:31-34 (in LXX Hebrews 31:31-34) in full which calls for little explanation or application to prove his point (verse Hebrews 8:13).

I will make (συντελεσω). Future active of συντελεω, old compound verb to accomplish as in Mark 13:4; Romans 9:28.

A new covenant (διαθηκην καινην). In Hebrews 12:24 we have διαθηκης νεας, but καινης in 1. Corinthians 11:25. Καινος is fresh, on new lines as opposed to the old (παλαιος) as in 2. Corinthians 3:6; 2. Corinthians 3:14; νεος is young or not yet old.