Deuteronomy 11:18

But lay these my words upon your heart, and upon your soul, and bind them, written upon tephillin, as a sign upon the upper part (wrist?) of your left hands, and let them be for tephillin over thy forehead between thy eyes.

The word frontlets has already been explained [comment on 6: 8]. If you take these matters to heart, you will not perish from upon the land; rather, you will extend your days as long as the heaven endures on the earth.

THEREFORE SHALL YE PUT THESE MY WORDS IN YOUR HEART. “Even after you have been exiled, be distinguished by the commandments. Put on the T’fillin, attach a Mezuzah [to the doorpost of your home], so that they will not be novelties to you when you return to the Land. And so Scripture states, Set thee up waymarks.”, Verse 21. Jeremiah lived in the time when the people were about to go into the Babylonian exile. His prophecy was thus a warning that they were to retain their distinctiveness through the commandments. This is Rashi’s language. Now I have already written on the meaning of this matter, that these commandments [the T’fillin and the Mezuzah mentioned by Rashi] are personal obligations and, as such, their laws apply in all places just as in the Land! But there is in this Midrash 43. Hence Ramban refers to it as “this Midrash.” a profound secret, and I have already alluded to it. Now he repeated these commandments about the T’fillin, the study of the words of the Torah, and the Mezuzah, a second time in order to allude to this analogy, that we shall be obligated to observe them after being exiled from the Land of Israel. From these [texts] we learn that all commandments that are personal obligations are binding in all places and that, outside the Land of Israel, we are exempt from duties which apply to ground, such as heave-offering and tithes. So it is interpreted in the Sifre. 44. And the intent of this interpretation is suggested by the fact that he placed the expression therefore shall ye put [these my words in your heart] next to [the verse], and ye shall perish [quickly from off the good Land thus clearly intimating that even after ye shall perish from off the good Land you shall still put these My words in your heart]. But the main reference of the verse [even regarding personal obligations] applies to the Land, as it is written, that your days may be multiplied, and the days of your children, upon the Land; or it may be that the verse is stating: “that your days may be multiplied, so that you will return [to the Land] and you will prolong your days on it forever.” is to the Land of Israel; this is evidenced by the concluding verse, that your days may be multiplied …upon the Land. According to the second interpretation the former verse has reference to the exile, while the second verse states the reward, that your days may be multiplied, so that you return and live upon the Land.
In line with the plain meaning of Scripture, it is possible that he comes to add here the expression to speak of them,.’ for there [in the first charge to the individual] he commanded and ‘thou’ shalt speak — you yourself — of them when ‘thou’ sittest in thy house, and here [in the charge to the community he stated that we are to teach the words of Torah to our children until they speak of them always. So also he added here and ye shall ‘teach’ them,.’ for the term v’shinantom [mentioned there] means only that he shall “tell” them the commandments, and here [he added that the obligation extends] until they teach the words of Torah [to their children] and they understand them and their reasons [well enough] to discuss them with you at all times. So also he added here as the days of the heavens above the earth which means “for all generations,” or it may refer to the first heaven and earth, and the days of old. The student learned [in the mysteries of the Cabala] will understand.

ושמתם את דברי אלה על לבבכם, to understand them through intensive study;

ועל נפשכם, so that you will observe them willingly.