Powtórzonego Prawa 32:18 Komentarz: Rashi, Sforno, Kitzur Baal HaTurim & Or HaChaim

תשי means THOU HAST FORGOTTEN. But our Rabbis explained it thus: whenever He was about to bestow good upon you, you provoked Him to anger and, as it were, weakened (תש) His power so that He could not do you good (Sifrei Devarim 319:3).

צור ילדך תשי, even you, Yeshurun, the one blessed with erudition, once you turned your attention to the physical enjoyments of life on earth, undermined, weakened your wisdom and your knowledge about the greatness of G’d and His Torah.

You disregard. The letter yud, with the value of 10, is small, to imply that Hashem gave you Ten Commandments, and you tested Him with ten trials, and also to imply that Hashem should remember Avraham whom he tested with ten tests.