In Exodus it says God hardens Pharaoh's heart. Isn't this unfair? I thought God didn't override our free will!
You are right about free will. God is sovereign and omniscient, but his power and knowledge never force us to do wrong. In Him there is no injustice.
Actually, in the Exodus, Pharaoh's heart is:
Said to be hard (7:13,14,22; 8:19; 9:17,35), or
He Hardens it himself (8:15,32; 9:34), or
God hardens it (4:21; 7:3; 9:12; 10:1,20,27; 11:10; 14:4,8).
Yet the original Hebrew uses the verb to be made strong in about half of the cases, and to be made heavy in many of the remaining instances. Technically speaking, the verb to be hardened occurs only once, in 7:3.
I like the analogy about the sun. It hardens clay, but melts butter. We react to God's influence according to our nature. Are we hard-hearted, stubborn? Then the circumstances of life, the things God sends our way, can stiffen us. Are we soft-hearted, humble? Then we tend to listen, to respond spiritually to input and challenges.
While it appears that Pharaoh and God alternately harden his heart, I believe the truth is that Pharaoh was the one in control. The alternation is only one of style; the two are not taking turns being peevish or unfair.