Moses and Aaron did all these wonders before Pharaoh, but the LORD hardened Pharaoh’s heart so that he would not let the Israelites go out of his land.
Warning is here given to Pharaoh of the last and conquering plague which was now to be inflicted. This was the death of all the first-born in Egypt at once, which had been first threatened (Exo 4:23, I will slay thy son, thy first-born), but is last executed; less judgments were tried, which, if they had done the work would have prevented this.
Commenting on Exodus 11:4-10
And Moses and Aaron did all these wonders before Pharaoh, Which are related in the preceding chapters: and the Lord hardened Pharaoh's heart: one time after another, and yet more and more: so that he would not let the children of Israel go out of his land; until the last plague, the slaying of the firstborn, was brought upon him and his people, related in the following chapter.