Carl Friedrich Keil and Franz Delitzsch Old Testament Commentary

Israel would then hearken again to the voice of the Lord and keep His commandments, and would rejoice in consequence in the richest blessing of its God. In the expression, ושׁמעתּ תשׁוּב אתּה ("thou shalt return and hearken"), תּשׁוּב ("thou shalt return") has an adverbial signification. This is evident from the corresponding expression in Deu 30:9, "for Jehovah will again rejoice over thee" (lit., "will return and rejoice"), in which the adverbial signification is placed beyond all doubt. Deu 30:8-10 contain the general thought, that Israel would then come again into its normal relation to its God, would enter into true and perfect covenant fellowship with the Lord, and enjoy all the blessings of the covenant. Deu 30:9 Deu 30:9 is a repetition of Deu 28:11. The Lord will rejoice again over Israel, to do them good (vid., Deu 28:63), as He had rejoiced over their fathers. The fathers are not the patriarchs alone, but all the pious ancestors of the people. Deu 30:10 A renewed enforcement of the indispensable condition of salvation.