A.T. Robertson Commentary Romans 6:10

"For the death that he died, he died unto sin once: but the life that he liveth, he liveth unto God." — Romans 6:10 (ASV)

The death that he died (ο απεθανεν). Neuter relative, cognative accusative with απεθανεν.

Once (εφαπαξ). Once and once only (Hebrews 9:26f.), not ποτε (once upon a time).

The life that he liveth (ο ζη). Cognate accusative of the relative.