God sent frogs all over Egypt

This could be translated as, “God caused many frogs to appear throughout Egypt.”

hardened his heart

He became stubborn again and refused to obey God. Here "heart" is a synecdoche that refers to Pharaoh. His stubborn attitude is spoken of as if his heart was hard. Alternate translation: "will cause Pharaoh to be stubborn" (See: rc://en/ta/man/translate/figs-synecdoche, rc://en/ta/man/translate/figs-metaphor)