Sh'mot 10:1-2 Then the L-RD said to Moshe, "Go to Pharaoh, for I have hardened his heart and the heart of his servants, that I may perform these signs of Mine among them, and that you may tell in the hearing of your son, and of your grandson, how I made a mockery of the Egyptians, and how I performed My signs among them; that you may know that I am the L-RD."
Why did G-d continue to harden Pharaohs heart? To teach the Egyptians that their gods were powerless compared to the true and living G-d. To teach the Israelites who their G-d was and that He is superior to any other god.
You may tell - The story of the Exodus is retold each Pesach (Passover).
Sh'mot 10:3-6 And Moshe and Aaron went to Pharaoh and said to him, "Thus says the L-RD, the G-d of the Hebrews, 'How long will you refuse to humble yourself before Me? Let My people go, that they may serve Me. 'For if you refuse to let My people go, Look, tomorrow I will bring locusts into your territory. 'And they shall cover the surface of the land, so that no one shall be able to see the land. They shall also eat the rest of what has escaped-- what is left to you from the hail-- and they shall eat every tree which sprouts for you out of the field. 'Then your houses shall be filled, and the houses of all your servants and the houses of all the Egyptians, something which neither your fathers nor your grandfathers have seen, from the day that they came upon the earth until this day.' "And he turned and went out from Pharaoh.
Pharaoh is given a warning before the 8th plague. This plague of locusts would be more sever than any previous plague.
Sh'mot 10:7 And Pharaoh's servants said to him, "How long will this man be a snare to us? Let the men go, that they may serve the L-RD their G-d. Do you not realize that Egypt is destroyed?"
Pharaoh's servants speak up and try to talk some sense into him. They remind him that the country is devastated.
Sh'mot 10:8-11 So Moshe and Aaron were brought back to Pharaoh, and he said to them, "Go, serve the L-RD your G-d! Who are the ones that are going?" And Moshe said, "We shall go with our young and our old; with our sons and our daughters, with our flocks and our herds we will go, for we must hold a feast to the L-RD." Then he said to them, "Thus may the L-RD be with you, if ever I let you and your little ones go! Take heed, for evil is in your mind. Not so! Go now, the men among you, and serve the L-RD, for that is what you desire." So they were driven out from Pharaoh's presence.
Pharaoh attempts to negotiate with Moshe. He wants to only let the men go into to worship G-d. This compromise would basically be the equivalent of holding the women and children hostage and would ensure that the men would return to Egypt.