Why does the Torah sometimes use the expression "va'Yechezak Lev...", and other times "va'Yachbed Es Libo"?
Rashbam (to 7:23) and Seforno 1 (to 8:11): Pharaoh was a strong man who was capable of hardening his heart even when the odds were stacked against him. Even he, however, had to assert himself to play it tough following the terrifying plagues of frogs and wild beasts. 2 Hence the Torah writes "va'Yachbed Es Libo." 3
Rashi (to 7:22) writes, that when Pharaoh later hardened his heart after the Plague of Blood, he did so with the assumption that Moshe and Aharon were using sorcery. Why doesn't Rashi make the same comment here?
Gur Aryeh (to 7:22): Aharon's staff swallowed up the other staffs only after it had reverted to wood; sorcerers cannot do that! Pharaoh hardened his heart here for a different reason - because so long as he was not yet stricken with any punishment, he preferred to say that he was ignoring Hashem and His message. Later, after the first plague hit, he came up with a rationalization so as not to attribute it to Hashem. 1
Gur Aryeh (ibid.): That is why verse 7:22 juxtaposes Pharaoh's hardening his heart, and his sorcerers' successful imitation of the plague.
