A.T. Robertson Commentary Hebrews 6:13

"For when God made promise to Abraham, since he could swear by none greater, he sware by himself," — Hebrews 6:13 (ASV)

Made promise (επαγγειλαμενος). First aorist middle participle of επαγγελλω. Could swear by none greater (κατ' ουδενος ειχεν μειζονος ομοσα). Imperfect active of εχω in sense of εδυνατο as often with ομοσα (first aorist active infinitive of ομνυω) and ωμοσεν (he sware) is first aorist active indicative.