Ex 7:3 "And I will harden Pharaoh's heart, and multiply My signs and My wonders in the land of Egypt.

4 "But Pharaoh will not heed you, so that I may lay My hand on Egypt and bring My armies and My people, the children of Israel, out of the land of Egypt by great judgments.

5 "And the Egyptians shall know that I am the LORD, when I stretch out My hand on Egypt and bring out the children of Israel from among them."

Moses is warned about Pharaoh's response in spite of Aaron's eloquence and Moses' ability to perform supernatural signs. Moses was a pragmatist. He was results-oriented. You are doing your job if you get results; you are a failure if you can't show any. Moses must realize that Pharaoh's stubborn heart is part of God's plan. God's ultimate purpose in all these is His glory. That "the Egyptians shall know that I am the LORD."