Charles Ellicott Commentary Deuteronomy 31:21

"And it shall come to pass, when many evils and troubles are come upon them, that this song shall testify before them as a witness; for it shall not be forgotten out of the mouths of their seed: for I know their imagination which they frame this day, before I have brought them into the land which I sware." — Deuteronomy 31:21 (ASV)

This song ... shall not be forgotten out of the mouths of their seed. —And it is not forgotten now. Saint Paul made special use of it in the last days of the second Temple. This song is a favorite piece of Hebrew poetry to this day. Rashi observes: “This is a promise to Israel that the law shall not be utterly forgotten by their seed.”

I know their imagination. —Hebrew, yêtzer, the same word employed in Genesis 6:5; Genesis 8:21. It is the word commonly used in Rabbinical literature for the evil nature or good nature in any man. The nature which they are forming, or making, this day, would be a literal rendering of the sentence in this verse.

And yet with all this, He made Balaam say, He hath not beheld iniquity in Jacob nor seen perverseness in Israel (Numbers 23:21). Compare to 1 Chronicles 28:9, The Lord ... understandeth all the imaginations of the thoughts, and Psalms 103:14, He knoweth our frame (yêtzer); He remembereth that we are dust.