A.T. Robertson Commentary Hebrews 8:4

"Now if he were on earth, he would not be a priest at all, seeing there are those who offer the gifts according to the law;" — Hebrews 8:4 (ASV)

On earth (επ γης). As opposed to εν τοις ουρανοις (verse 1). Condition of second class, determined as unfulfilled.

He would not be a priest at all (ουδ' αν ην ιερευς). "Not even would he be a priest." Conclusion of second class condition with αν and imperfect indicative (ην).

Seeing there are those (οντων των). Genitive absolute with οντων (from ειμ) and the articular present active participle of προσφερω (verse 3). Jesus was not of the tribe of Levi and so could not serve here.