Now therefore write ye this {i} song for you, and teach it the children of Israel: put it in their mouths, that this song may be a witness for me against the children of Israel.

(i) To preserve you and your children from idolatry, by remembering God's benefits.

Wesley's Notes

31:19 Write this song - Which is contained Deu 32:1 - 43, and is put into a song that it may be better learned, and more fixed in their minds and memories. Put it in their mouths - Cause them to learn it, and sing it one to another, to oblige them to more circumspection. A witness - Of my kindness in giving them so many blessings, of my patience in bearing so long with them, of my clemency in giving them such fair and plain warnings, and my justice in punishing such an incorrigible people.

Jamieson-Fausset-Brown Bible Commentary

19. Now therefore write ye this song-National songs take deep hold of the memories and have a powerful influence in stirring the deepest feelings of a people. In accordance with this principle in human nature, a song was ordered to be composed by Moses, doubtless under divine inspiration, which was to be learnt by the Israelites themselves and to be taught to their children in every age, embodying the substance of the preceding addresses, and of a strain well suited to inspire the popular mind with a strong sense of God's favor to their nation.

Matthew Henry's Concise Commentary

31:14-22 Moses and Joshua attended the Divine Majesty at the door of the tabernacle. Moses is told again that he must shortly die; even those who are most ready and willing to die, need to be often reminded of its coming. The Lord tells Moses, that, after his death, the covenant he had taken so much pains to make between Israel and their God, would certainly be broken. Israel would forsake Him; then God would forsake Israel. Justly does he cast those off who so unjustly cast him off. Moses is directed to deliver them a song, which should remain a standing testimony for God, as faithful to them in giving them warning, and against them, as persons false to themselves in not taking the warning. The word of God is a discerner of the thoughts and intents of men's hearts, and meets them by reproofs and correction. Ministers who preach the word, know not the imaginations of men; but God, whose word it is, knows perfectly.