Signs, Wonders, and Punishment

What do the three phrases יד חזקה וזרוע נטויה ושפטים גדולים mean? What is the difference between אות and מופת?

Why does Hashem harden Pharaoh's heart? According to the Rambam Hashem removes his freewill in order to facilitate punishment. What would have happened if Pharaoh gave in earlier? According to the Seforno the plagues served to remove Pharaoh's free will. If Pharaoh had given in earlier, he would not have received his just deserts.

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Delivered at the OU Israel Center, January 3rd, 2019 (26 Tevet 5779)