A.T. Robertson Commentary Romans 9:15

"For he saith to Moses, I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion." — Romans 9:15 (ASV)

For he says to Moses (τω Μωυσε γαρ λεγε). He has an Old Testament illustration of God's election in the case of Pharaoh (Exodus 33:19).

On whom I have mercy (ον αν ελεω). Indefinite relative with αν and the present active subjunctive of ελεαω, late verb only here and Jude 1:23 in N.T. "On whomsoever I have mercy." The same construction in ον αν οικτειρω, "on whomsoever I have compassion."