John Gill Commentary Deuteronomy 4:14

"And Jehovah commanded me at that time to teach you statutes and ordinances, that ye might do them in the land whither ye go over to possess it." — Deuteronomy 4:14 (ASV)

And the Lord commanded me at that time
When the ten commandments were delivered on Mount Sinai, and Moses was ordered to come up to God in the mount:

to teach you statutes and judgments ;
laws ceremonial and judicial, besides the ten commands given them:

that you may do them in the land where you go over to possess it ;
the land of Canaan, which was on the other side of Jordan, and over which they must go in order to possess it; and when they came there, they were to hold the possession of it by attending to those laws which forbad the sins for which the old inhabitants of it were expelled out of it; and besides these, there were also several laws, both ceremonial and judicial, which were to be peculiarly observed in the land, as well as others they were obliged to do while without it.