Moses is very old, and will not cross the Jordan; but the Lord will cross the Jordan and crush the Canaanites, with Joshua leading the people. Every seven years, the Law will be taught to the people by the priests. It will also be taught by elders and public officials. The Lord appears to Moses, warning him that the Israelites will rebel against Him, incurring His wrath. He knew this would happen even before making the agreement with the Israelites. This is not a failure of the Law, but just part of a plan that will be fulfilled by Christ. The Lord tells Moses to teach the Hebrews a song as a reminder of the Law after they have forsaken it. If the Hebrews try to argue that the Lord has not held up His end of the bargain, this song is a rebuttal.