“I know that the king of Egypt will not give you permission to go.” God knew both the heart of Pharaoh and the culture of the time, so it made sense to Moses that God would know Pharaoh would not let his slaves go just because Moses asked him to. God knew there would have to be a power-showdown between He and Pharaoh, but it was still Pharaoh’s freewill choice to not let the Israelites go when Moses started displaying the power of God. God started demonstrating His power very gently, with no loss of life of man or beast. Only as Pharaoh continued to harden his heart did the plagues get really damaging.
“no, not unless compelled by a mighty hand.” The Hebrew phrase is difficult, and most translations do what the REV has done and clarify the text by adding words about Pharaoh being compelled or forced to let Israel go. However, it is possible that at this early time God is warning Moses about how difficult it will be to get Pharaoh to let Israel go. If that is the case, then God is saying that Pharaoh will not let Israel go, no, not even by a mighty hand, i.e., force. That turned out to be true. In plague after plague Pharaoh stubbornly refused to let Israel go.