en_tn/exo/04/21.md

will harden his heart

Possible meanings are 1) "will make him unwilling to obey you" or 2) "will make him unwilling to obey me."

Israel is my son

The word "Israel" is a shortened form of "the people of Israel." (See: rc://en/ta/man/translate/figs-metonymy)

is my son, my firstborn

"is like my son, my firstborn" or "is like a firstborn son to me" (See: rc://en/ta/man/translate/figs-metaphor)

you have refused to let him go

The word "him" refers to the people of Israel as God's son. (See: rc://en/ta/man/translate/figs-metonymy)

I will certainly kill your son, your firstborn

The word "son" here refers to the actual son of Pharaoh.