The Torah warns us to not forget the revelation that we witnessed at Har Sinai, for as the pasuk in Parshas Va’eschanan says: “Only beware for yourself and greatly beware for your soul, lest you forget the things that your eyes have seen and lest you remove them from your heart all the days of your life, and make them known to your children and your children’s children” (Devarim 4:9).
There seems to be several different interpretations of this pasuk. The Mishnah in Pirkei Avos (3:8) says that anyone who forgets what he has learned is considered to be deserving of death. The Mishnah quotes the abovementioned pasuk as a reference. Similarly the Gemara in Menachos 99b says in the name of Reish Lakish that someone who forgets what he has learned has transgressed the lav of the aforementioned pasuk. The Gemara explains that since the pasuk used the words “hi’shamer” and “pen,” this pasuk constitutes a negative commandment.
The Gemara in Kiddushin 30a derives from this pasuk that one who learns Torah with his son’s son is considered to have received the Torah at Har Sinai.
However, at the same time, the literal translation of the pasuk is that we should not forget what we saw on Har Sinai. How does the literal translation lend itself to the prohibition of not forgetting one’s learning and praising one who learns with his grandchildren?
Another point of discussion is whether the lav of not forgetting the revelation that we witnessed at Har Sinai is on the list of the 613 mitzvos. The Rambam does not include this in his list of negative commandments. The Ramban, in his commentary to the Rambam’s Sefer Hamitzvos (in the section of the prohibitions that the Rambam neglected to count, in mitzvah 2), writes that this pasuk serves as a prohibition for one to forget ma’amad Har Sinai and that the Rambam neglected to count it. According to the Ramban, if one forgets the revelation that we witnessed at Har Sinai he has transgressed a lav.
The sefer, Megillas Esther (commentary to the Ramban’s commentary to the Rambam’s Sefer Hamitzvos), explains that the Rambam did not count as a negative commandment to forget the revelation that we witnessed at Har Sinai because he understood that the pasuk is referring to forgetting the Torah one has learned. This opinion is shared by other Rishonim, such as the Yereim (359) and the Sefer Mitzvos Ketanos (96), who posit that there is no negative commandment to forget ma’amad Har Sinai; rather, the pasuk is only prohibiting the forgetting of the Torah that one has learned – as the Mishnah in Avos and Gemara in Menachos mention. The sefer, Megillas Esther, explains that the Rambam did not count the prohibition to not forget one’s learning as a separate mitzvah, for it is part of the mitzvah of learning Torah.
The question still remains on how to reconcile all of the different drashos that are derived from this pasuk. The truth is that the Ramban actually asks this question on himself. He says that one should not be mistaken and think that the Gemara in Kiddushin 30a, which praises one who learns Torah with his grandchildren, based on this pasuk, understands that this pasuk is only referring to learning Torah and not remembering matan Torah. The Ramban explains that the Gemara is referring to learning about emunas haTorah (belief in the Torah), which is learning Torah as well.
Perhaps the explanation of these words is as follows: There is another aspect to the mitzvah of learning Torah. Aside from learning Torah, one must acquire emunas haTorah, believing that our Torah is the word of God that we received on Har Sinai. It is regarding the second aspect of the mitzvah that the Gemara in Kiddushin says that one who learns with his grandchildren is considered to have personally accepted the Torah on Har Sinai. This is because when one learns Torah with his grandfather, he is learning with someone from one generation closer to Har Sinai. This will instill in him a greater sense of emunas haTorah. This learning has both aspects of the mitzvah in it. Therefore, the Gemara reveres a grandfather who teaches Torah to his grandchildren; it’s as if he has accepted the Torah on Har Sinai.