John Gill Commentary Exodus 10:20

"But Jehovah hardened Pharaoh`s heart, and he did not let the children of Israel go." — Exodus 10:20 (ASV)

But the Lord hardened Pharaoh's heart
For as yet he had not brought all his judgments on him he designed to bring:

so that he would not let the children of Israel go :
though he had promised to do it, and that he would never offend more in this way.