The phrase "hardness of heart" is a metaphor that means "stubbornness." Alternate translation: "Because of your stubbornness" or "Because you are stubborn" (See: [[rc://en/ta/man/translate/figs-metaphor]])
Here "you" and "your" are plural. Jesus is speaking to the Pharisees, but Moses gave this command many years earlier to their ancestors. Moses' command applied to all Jewish men in general. (See: [[rc://en/ta/man/translate/figs-you]])
# from the beginning
Here "beginning" refers to when God first created man and woman. (See: [[rc://en/ta/man/translate/figs-metonymy]])