Why did Hashem harden Par'oh's heart? And once He did, how could He then continue to punish him?
Seforno: Refer to 10:1:2:1.
R. Bachye: To teach him that He holds the heart of the people and of kings 1 in His Hands to do with it as He pleases ? 'like putty in the hands of the potter' 2 .
As Shlomoh ha'Melech wrote in Mishlei ""Palgei Mayim Leiv Melech be'Yad Hashem, el Kol asher Yachpotz Yatenu!"
R. Bachye: As Yirmiyah wrote explicitly in 18:6, regarding Yisrael.