A.T. Robertson Commentary Hebrews 10:12

"but he, when he had offered one sacrifice for sins for ever, sat down on the right hand of God;" — Hebrews 10:12 (ASV)

When he had offered (προσενεγκας). Second aorist active participle (with first aorist ending -ας in place of -ον) of προσφερω, single act in contrast to present participle προσφερων above.

One sacrifice (μιαν θυσιαν). This the main point. The one sacrifice does the work that the many failed to do. One wonders how priests who claim that the "mass" is the sacrifice of Christ's body repeated explain this verse.

For ever (εις το διηνεκες). Can be construed either with μιαν θυσιαν or with εκαθισεν (sat down). See 1:3 for εκαθισεν.