>difference between the Old and New testaments
In a nutshell.
God chose for himself a peoples, the descendants of Abraham, Isaac and Jacob, made a covenant with them, and started a country and ruled over them. He gave them the Law, 613 in total, but the 10 commandments cover the basics. The covenant was keep the Law and I will bless you; violate the Law and I will curse you. Keep the land refreshed or you will lose it.
The Law was given to mankind because mankind followed the devil's gospel: eat the fruit, know good and evil, and thus be like God. God gave the Law to show mankind the first step to being like Him. And that first step was impossible.
400 years after God stopped enforcing the Old Covenant, Jesus was born. Jesus kept the Law perfectly, and Jesus paid the sin debt of humanity for all of humanity's violations of the Law. God's holiness, justice and righteousness thus satisfied, God in His grace and mercy offered mankind a New Covenant, completely unlike the Old.
This New Covenant was a gift to mankind, free for the taking, and enabled whoever believes in Jesus to have the power to become a son of God. To become a brother or sister to Jesus, and a joint heir. To know the living God. To live with God in heaven forever.
Nothing in the Old Covenant comes close to that. The blessings in the Old Covenant, which were never deserved, were to have more flour in your kneading bowl, to have bumper crops, to not have mold and mildew eat away your belongings, to not have your children attacked by animals, and to have God defeat your enemies. All blessings here on earth, and no relationship with the living God.
So under the New Covenant, God has taken for Himself a people zealous for good deeds, who desire the inheritance that comes with being adopted into the family of God, and who want to live with God in heaven forever.
Two different deals/covenants.
Two different parties on the other end.
Two different sets of blessings/curses.