Gemara

(Reish Lakish): One who forgets something that he learned transgresses a Lav - "Hishamer Lecha u'Shmor Nafshecha Me'od Pen Tishkach Es ha'Devarim..."

(R. Avin): Wherever it says "Hishamer", "Pen" or "Al", this is a Lav.

(Ravina): It says here "Hishamer" and "Pen". There are two Lavim.

(Rav Nachman bar Yitzchak): He transgresses three Lavim. It says also "u'Shmor Nafshecha Me'od."

Suggestion: Perhaps he transgresses even if he forgot due to Ones!

Rejection: "U'Fen Yasuru mi'Lvavecha" - he is liable only if he removes them from his heart.

Suggestion (R. Dostai b'Rebbi Yanai): Perhaps he transgresses even if his learning was too great for him to remember!

Rejection: "Rak" (excludes that case).

Avodah Zarah 19a (Rava): One should learn the text, even if he forgets or does not understand it.

(Rava citing Rav Sechorah): "Hon me'Hevel Yim'at v'Kovetz Al Yad Yarbeh" - if one learns large amounts at a time, without time to review properly, he will lose it. If he learns small amounts at a time, he will grow.

Rava: Many Rabanan know this, yet transgress it.

(Rav Nachman bar Yitzchak): I fulfilled the teaching and remembered my learning.

(Rav Shizbi): "Lo Yacharoch Remiyah Tzeido" teaches that one who deceives (he projects himself to be a Chacham because he has memorized much text) will not (remember it, nor) live a long time.

(Rav Sheshes): It teaches that one who learns small amounts at a time (and reviews it often) will surely have what to eat (remember his learning)!

(Rav Dimi): A parable explains this. If a hunter breaks the wings of each bird he catches before hunting the next, he will keep all of them. If not, he will lose them.

Bava Basra 10b: R. Yehoshua's son Yosef passed out, and recovered. He said that they were saying 'happy is the one who comes to here (the world to come) and remembers his learning.'

Sanhedrin 26b (Ula): Thoughts help even for Torah - "Mefer Machshevos Arumim v'Lo Sa'asenah Yedeihem Tushiyah."

(Rabah): This does not apply to if he learns Lishmah - "Rabos Machashavos b'Lev Ish va'Atzas Hash-m Hi Sakum."

99a (Beraisa - R. Yehoshua): One who learns Torah and forgets it is like a woman who bears children and buries them.

R. Akiva says, if one constantly reviews his learning, he will remember it, like one remembers a song that he always sings.

(R. Yitzchak bar Avodimi): He learns from "Nefesh Amel Amlah Lo Ki Achaf Alav Pihu."

Koheles Rabah (3:10): R. Yitzchak says, it is good that people forget Torah. If not, one would learn from two or three years, then engage in his work, and never engage in Torah again. Since he forgets, he never ceases to learn Torah.

Medrash Mishlei (10 DH Amar R. Yishmael): R. Yishmael said, the day of judgment is harsh. When a Chacham comes and says that he engaged in Torah, Hash-m says 'say in front of Me what you learned.' This is the source for Chachamim's counsel 'whatever one learned should be secure in his hand', lest he be embarrassed in the future.

Rishonim

Shitah Mekubetzes (99b): (The text is) a Chacham who forgets transgresses.

Rashi (Sanhedrin 26b DH Machshavah): Worries about finances help one to forget his learning.

Aruch (Erech Takaf): We suggested that one transgresses even if his learning was too difficult for him to remember (and rejected this)

R. Gershom (DH Yachol): We suggested that he transgresses even if he forgot it because he never understood it.

Iyun Yakov (99b DH Davar): The verse says "lest you forget ha'Devarim." This connotes all or most of Torah. Why is one liable for forgetting one thing? "Im Shacho'ach Tishach" teaches that if you begin to forget, in the end you will forget everything. This is why the Torah said "be very careful." Also, many matters depend on one thing. We suggested that he is liable even if he forgot due to Ones, because forgetting learning is considered like Mezid (Avos 4:12). Had he been zealous, he would have remembered everything.

Etz Yosef (99b DH Yachol): At first, we suggested that he is liable even for forgetting due to Ohnes of illness, like Rav Yosef who became sick and forget his learning.

Tosfos Yom Tov (Avos 3:8): He is liable only if he removes them from his heart, i.e. he does not review his learning.

Maharsha (Avodah Zarah 19a DH Mashal): Many Rabanan know that one who learns large amounts at a time, without time to review properly, he will lose it. Still, they transgress it, for they follow Rava's teaching to learn the text, even if he forgets or does not understand it.

Sefer Chasidim (1008): Mice ate from Ploni's bread. He asked a Chacham if he may eat it, for he is hungry, but he is commanded not to eat what will cause him to forget his learning. The Chacham answered 'you are liable only if you remove Divrei Torah from your heart. I see that the entire day you are Batel from Torah, and accompany Amei ha'Aretz to hear idle words. It would be better if you did not avoid things that cause forgetting, so you would forget the idle words you engage in'!

Sefer ha'Zikaron (of ha'Gaon R. C. Kanievsky Shlita, in Si'ach ha'Sadeh, part 2, introduction): We learn from this that there is not even an Isur mid'Rabanan unless one removes Divrei Torah from his heart. All the Poskim did not bring things that cause one to forget. This implies that they are not forbidden. Why didn't they explain what is called 'removing from one's heart'? We must say that since they do not surely cause him to forget, rather, they merely weaken memory, and he eats them with intent for his benefit, (this is not forbidden). However, if one is Batel and does not review his learning, surely he will forget, this is considered removing it from his heart. Surely it is proper to refrain from things that cause to forget. This is why Chachamim listed them!

Yereim 359 (28 in old editions): One is liable for forgetting Torah if he removes it from his heart, i.e. he turns his heart to idleness. He transgresses a Lav, and it is considered that he is Chayav Misah.

Derech Mitzvosecha (2 Azharah 9, cited in Shmiras ha'Guf veha'Nefesh introduction 17): The Rambam omitted it because it is a general command. He said in Shoresh 4 that it is improper to list such commands.

Poskim

Shulchan Aruch (YD 246:4): After one matures in Torah, he need not learn written Torah, nor engage constantly in oral Torah (Mishnah). He should have set times to read Torah, Nevi'im and Kesuvim and oral Torah, lest he forget laws of Torah, and he fully focuses on Talmud.

Prishah (6): If one does not learn, he removes Divrei Torah from his heart. Only then he is liable for causing himself to forget. One is exempt if he forgets automatically.

Sefer ha'Zikaron: Medrash Mishlei (10 DH Lo Yar'iv) says that in the future, they will help one to remember everything that he learned. "Lo Yar'iv Hash-m Nefesh Tzadik" refers to a Chacham. If he learned Torah in his lifetime, and forgot a matter or chapter of his learning, before he dies, angels arrange it for him, lest he be embarrassed in front of Hash-m. This is only if he reviewed his learning and did what he could to remember, but due to Ones he forgot. If he was lazy and did not review properly, also afterwards he will not remember, like it says in Bava Basra.

Shulchan Aruch ha'Rav (Hilchos Talmud Torah 2:4): One who forgets Torah because he did not review transgresses "Hishamer Lecha." This is even nowadays that oral Torah was written. One transgresses immediately, even if afterwards he will look up the law, just like one used to transgressed even if he would ask his Rebbi afterwards. Also, laws of the Torah do not change.

Shulchan Aruch ha'Rav (ibid., 3 Kuntres Acharon): There is no Isur to refrain from marrying, like Ben Azai did. Even though we interrupt Torah for a Mitzvah that cannot be done through others, this refers to a Mitzvah that takes time, and then one returns to learn. Marrying entails financial obligations (to feed one's family), which is a permanent Bitul Torah. This can lead to forgetting Torah, if he lacks time to review. Peru u'Rvu does not override knowledge of Torah, just like the Gemara said that at the beginning it does not override it (one should learn first and then marry). This is because knowledge of Torah is greater than fulfilling (Mitzvos of) Torah. The Rambam holds that Ben Azai never married, lest he cease engaging in Torah and forget it. One is Chayav Misah and transgresses a Lav for forgetting even one matter. The Rambam did not list it among the 613 Mitzvos because he holds that it is part of the Aseh of Talmud Torah. (Similarly, he did not list "Hishamer Lecha" of Tefilin (not to wear them at night), for it is part of the Aseh of Tefilin.) Perhaps he holds that forgetting due to not reviewing properly is 'his learning was too much.' He is liable only if he removes them from his heart. However, it is difficult to say that he explains unlike the simple meaning and all the other Meforshim. Also, Chachamim cursed the swindler (who does not review his learning). The Semak counted "Hishamer Lecha" among the Mitzvos. He holds that it is a real Drashah.

Shulchan Aruch ha'Rav (ibid.): (The Rambam holds that) Chachamim do not argue with Ben Azai. They married, for if one forgets because he must support a family and lacks time to review, he does not transgress, for this is Ones. We do not neglect a Chiyuv (Peru u'Rvu) due to a Safek lest one be Mevatel knowledge of Torah. Rashi holds that Pilpul (deep investigation) was given only to Moshe; there is no Isur to forget it. R. Shimon used to give 24 answers to every question of R. Pinchas ben Ya'ir. Reish Lakish would ask 24 questions against every teaching of R. Yochanan, and R. Yochanan gave 24 answers. Only a tiny fraction of this remains. One is liable for forgetting even Kodshim and Taharos, even though nowadays they do not apply. This proves that one is obligated to learn them. R. Shimon said that one who plants, plows, harvests... does not fulfill the desire of the Torah. This refers to one who could learn and remember. One needs help from Shamayim to remember. Not everyone merits this. Some would forget even if they learned constantly.