The prohibition to forget one’s Torah learning:^[1]

It states in the Mishnah in Avos^[2], “Whoever forgets even a single teaching from his study, is considered by Scripture to be liable for his life, as the verse^[3] states, “Make sure to guard yourselves and guard your souls very much, lest you forget the words that your eyes saw.” The Talmud^[4] teaches us based on this verse, that whoever forgets even one matter from his studies transgresses a negative command. There are the various opinions in Rishonim regarding the severity of one’s forgetting of his Torah learning.

1. The opinion of the Rambam: The Rambam^[5], as well as many others who enumerate the list of the 613 commands, omit the above prohibition of forgetting one’s Torah learning from the list of 365 negative commands. This seems to imply that it is not a prohibition at all, and at least not a biblical prohibition.
2. The opinion of the Ramban: The Ramban^[6], unlike the Rambam, does enumerate the verse of “thou shall not forget..” as part of the 613 commands. However, as he understands it, this prohibition is not referring at all to forgetting one’s Torah learning but rather to forgetting the giving of the Torah on Mount Sinai. Thus, he too does not list the forgetting of one’s Torah learning as a biblical prohibition. According to the Ramban, it seems that he understands the statement in the above Mishneh in Avos and Talmud which seems to prohibit forgetting one’s Torah learning, to simply be a matter of piety which is supported by the above verse in Scripture, although it is not an actual obligation.
3. The prohibition is only when one stops learning Torah altogether: Indeed, there are a number of Rishonim who enumerate the above prohibition of forgetting one’s Torah learning as part of the 613 commands, hence accepting the above Mishnaic and Talmudic teaching as literal.^[7] Nonetheless, even they describe the prohibition is only applicable to one who completely drops his Torah learning, and get themselves involved in trivial matters which causes his Torah learning to become forgotten.
4. The opinion of the Alter Rebbe:^[8] The Alter Rebbe has a very unique and detailed approach in his understanding of the above Mishnaic and Talmudic prohibition of forgetting one’s Torah learning. In his laws of Torah study, the Alter Rebbe states based on Rabbeinu Yonah^[9],  that whoever forgets one aspect of his Torah learning due to him not reviewing it properly is considered by Scripture to be liable for his soul, and likewise transgresses a negative command in the Torah. Meaning, that not only does he hold that forgetting one’s Torah learning is under a biblical prohibition, but that it is transgressed even if one does not actively do anything to make him forget it, but simply is not active enough to retain the information that he acquired, such as if he does not review his learning. Even in today’s times that all the laws have been written in books, and anything that one has forgotten can be researched, nonetheless, the moment that one forgets his learning he transgresses the above negative command prior to looking it up. In his Kuntrus Achron^[10], the Alter Rebbe explains this to be likewise the opinion of the Rambam who more than once mentions of the idea of not forgetting one’s Torah learning.
5. The opinion of the Rebbe:^[11] Even according to the approach of the Alter Rebbe in his understanding of the prohibition against forgetting Torah, the prohibition is specifically in a case that one actively does something to neglect Torah learning in general. If, however, he does not actively neglect Torah learning and simply avoids reviewing certain sections of Torah due to lack of time, then since he has not neglected Torah learning therefore no prohibition is considered to have taken place.

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[1] See Likkutei Sichos Vol. 34 Parshas Vaeschanon; Divrei Avos 3:8 and Divrei David Avos 3 of Ashel Avraham of Butchach; Sefer Shemiras Haguf Vihanefesh [Lerner] Mavo 17

[2] Avos 3:8

[3] Vaeschanon 4:9

[4] Menachos 99b

[5] See also Rambam Talmud Torah 1:10, 12, brought in Kuntrus Achron Talmud Torah 3

[6] In Hashmatos of Sefer Hamitzvos L.S. 2

[7] See Semag Lavin 13; Semak 15; Yireim 28

[8] Admur Hilchos Talmud Torah 2:4

[9] See Rabbeinu Yonah Avos ibid.

[10] Beginning of chapter 3

[11] Likkutei Sichos Vol. 34 Parshas Vaeschanon