וְיָדַעְתָּ֣ הַיּ֗וֹם וַהֲשֵׁבֹתָ֮ אֶל־לְבָבֶךָ֒ כִּ֤י יְהוָה֙ ה֣וּא הָֽאֱלֹהִ֔ים בַּשָּׁמַ֣יִם מִמַּ֔עַל וְעַל־הָאָ֖רֶץ מִתָּ֑חַת אֵ֖ין עֽוֹד׃
Karena itu, ketahuilah pada hari ini dan camkanlah dalam hatimu bahwa Yang Mahakekal, Dialah Allah di langit di atas dan di bumi di bawah; tidak ada yang lain.
Sforno
וידעת היום, it behooves you therefore to understand how these miracles confound any theological theories that are not in agreement with Jewish tradition.
Or HaChaim
וידעת היום, "and you should know this day, etc." What is it that the people should know about on this day rather than on any other day? What knowledge were they to store up in their hearts? Perhaps Moses referred to the need to subdue the evil urge which lurks within all of us. Moses may have referred to 10 such pieces of information which the Israelites were to store up in their hearts.
Rabbeinu Bahya
והשבות אל לבבך, “you will impress it upon your heart,” i.e. it is a positive commandment to know the Lord and to research the fact that He is unique, one of a kind. It is not enough to accept this merely as “something handed down by our forefathers,” but it must become something that is the result of our examining the plentiful evidence in our world which makes this quite clear.