79 The clause beginning with the preterite and vav (ו) consecutive is here subordinated to the next, and main clause—that he hardened his heart again.
80 The construction is another verbal hendiadys: וַיֹּסֶף לַחֲטֹּא (vayyosef lakhatto’), literally rendered “and he added to sin.” The infinitive construct becomes the main verb, and the Hiphil preterite becomes adverbial. The text is clearly interpreting as sin the hardening of Pharaoh’s heart and his refusal to release Israel. At the least this means that the plagues are his fault, but the expression probably means more than this—he was disobeying Yahweh God.
81 This phrase translates the Hebrew word כָּבֵד (kaved); see S. R. Driver, Exodus, .

In other versions

But when Pharaoh saw that the rain and the hail and the thunder had ceased, he sinned yet again and hardened his heart, he and his servants.

When Pharaoh saw that the rain and hail and thunder had stopped, he sinned again: He and his officials hardened their hearts.

When Pharaoh saw that the rain, hail, and thunder had ceased, he sinned again and hardened his heart, he and his officials.

But when Pharaoh saw that the rain, hail, and thunder had stopped, he and his officials sinned again, and Pharaoh again became stubborn.

And Pharaoh saw that the rain and the hail and the thunder stopped, and he again sinned and made his heart insensitive, he and his servants.

And when Pharaoh saw that the rain and the hail and the thunders were ceased, he sinned yet more, and hardened his heart, he and his servants.

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