When Yahweh had Moses perform miracles, Pharaoh's men were able to copy these miracles. It is unknown how they were able to do this, but since it was not from Yahweh, they were probably done under some evil power. (See: [[rc://en/tw/dict/bible/kt/miracle]] and [[rc://en/tw/dict/bible/kt/evil]])
Pharaoh's heart is often described as hard in this chapter. This means that his heart was not open or willing to understand Yahweh's instructions. When his heart was hardened, it became less and less receptive to Yahweh.
This is a very important statement. Moses does not ask Pharaoh for permission to "let go" of the Hebrew people. Instead, he is demanding that Pharaoh free the Hebrew people.