Exodus 7:1-13 -- Pharaoh's Heart Hardened

Exodus 7:1 And the Lord said to Moses, “See, I have made you like God to Pharaoh, and your brother Aaron shall be your prophet. 2 You shall speak all that I command you, and your brother Aaron shall tell Pharaoh to let the people of Israel go out of his land. 3 But I will harden Pharaoh's heart, and though I multiply my signs and wonders in the land of Egypt, 4a Pharaoh will not listen to you. 

 So, here God told Moses that Pharaoh would not listen to him.  It did not matter if Moses could speak well and very eloquently or if he spoke broken and shuddered sentences, Pharaoh was not going to listen.  Moses's excuse of having "uncircumcised lips" did not matter because God had already hardened Pharaoh's heart.

Exodus 7:4b Then I will lay my hand on Egypt and bring my hosts, my people the children of Israel, out of the land of Egypt by great acts of judgment. 5 The Egyptians shall know that I am the Lord, when I stretch out my hand against Egypt and bring out the people of Israel from among them.”

Why did God not simply allow his people to leave Egypt and go without bringing the harsh judgments on Egypt? Well, this verse answers that question: so the Egyptians will know that He is God.

Exodus 7:6 Moses and Aaron did so; they did just as the Lord commanded them. 7 Now Moses was eighty years old, and Aaron eighty-three years old, when they spoke to Pharaoh.
8 Then the Lord said to Moses and Aaron, 9 “When Pharaoh says to you, ‘Prove yourselves by working a miracle,’ then you shall say to Aaron, ‘Take your staff and cast it down before Pharaoh, that it may become a serpent.’” 10 So Moses and Aaron went to Pharaoh and did just as the Lord commanded. Aaron cast down his staff before Pharaoh and his servants, and it became a serpent. 11 Then Pharaoh summoned the wise men and the sorcerers, and they, the magicians of Egypt, also did the same by their secret arts. 12 For each man cast down his staff, and they became serpents. But Aaron's staff swallowed up their staffs.13 Still Pharaoh's heart was hardened, and he would not listen to them, as the Lord had said.

Here, Moses and Aaron show Pharaoh they are from God by turning their staff into a snake, but Pharaoh's officials were able to do the same thing with their "secret arts."  Pharaoh still did not listen even when Moses's snake devoured all the other snakes.  The God of Israel was showing that he would over power the Egyptians by having the snake he created from the staff devour the others.