This verse is the source from which we learn that a father is obligated to teach his children Torah. As the Alter Rebbe writes:
A father is obligated to teach his young son Torah... as it is written, ‘You shall teach them to your sons to speak about them.’ From when is he obligated to teach him? From when he begins to speak, he should teach him Torah tzivah lanu Moshe..., as well as the first verse of Shema Yisrael.
Earlier, the Torah instructs us, “These words that I command you today... you shall teach them thoroughly to your children.” That command, however, speaks of a more advanced level of learning—when the child is capable of thoroughly grasping and retaining the Torah that he is taught. In addition, our Sages interpret the word “children” in that verse as a reference to students, one’s spiritual children. The mitzvah to “teach them thoroughly to your children” is the obligation upon the learned to teach the Torah to all who desire to study—even those who are not their biological offspring.
In contrast, the mitzvah given here, “You shall teach them to your children to speak with them,” refers to a very early stage in a child’s education, when he is just beginning to speak. As Rashi explains, this verse is meant quite literally: “From the moment your son knows how to speak, teach him [the verse] Torah tzivah lanu Moshe. Let him learn to speak with this.” This training obviously begins at home, long before the child is old enough to be sent to school. Clearly, then, this verse refers to a parent’s personal mitzvah to teach the Torah to his child—an obligation that goes into effect as soon as the child begins to talk.
—Likkutei Sichos, vol. 9, p. 33, fn. 3; Sichos Kodesh 5737, vol. 2, pp. 387–388
