Deuteronomy 33:4,5

(4,5)

“[Of] the law which Moses commanded us,

The inheritance of the congregation of Jacob,
When he (Moses) was king in Jeshurun,
In the gathering of the heads of the people,
The tribes of Israel together.”

This fourth verse, from its form, is evidently not what Moses said, but an explanatory parenthesis, inserted by the writer, who was probably Joshua. Upon “He was king in Jeshurun,” Rashi says, “The Holy One, blessed be He! the yoke of His kingdom is upon them for ever.” It may be so. “When the Lord your God was your king,” is Samuel’s description of the whole history of Israel previous to himself.
The certainty that the King of kings, the Messiah of Israel, was and is the Lawgiver and Teacher, and Keeper of all saints, and that there are none of that character who do not “sit at the feet of Jesus,” makes the real meaning of the passage perfectly plain, even though the exact grammatical relation of the clauses may be not beyond dispute.