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It is imperative upon a person to use his free time for Torah study. If one wastes one's free time on useless means, one is in violation of Bitul Torah.
Not Wasting Time from Learning Torah
- Even though one already learned one’s daily amount of learning, one is not exempt from learning if one can do so fruitfully. 
- Wasting time, when one could learn Torah, is a grievous sin and there are many punishments mentioned by Chazal for this sin. 
- In a case where a person has the opportunity to either learn torah or do a different mitzvah; if the mitzvah can be done by someone else, one may not interrupt his learning. If the mitzvah can not be done by anyone else, one should go do the mitzvah and then return to one's learning.
- If one needs to sleep during the day in order to learn Torah, one may do so.
- One who learns Torah correctly, should notice his knowledge and wisdom expanding as he grows. If one notices his learning is weakening not due to the toil of daily life, he may attribute it to some degree of bitul torah (not enough time spent learning, not enough mental and/or physical involvement, etc).
- Saying Shema in its time in order to fulfill the mitzvah of Kriyat Shema is a great mitzvah of Shema but according to many achronim isn't simultaneously considered learning Torah. Similarly, saying Tehillim as a tefillah is a great mitzvah but according to many achronim isn't considered learning Torah. On the other hand, some hold that it is learning if one understands what one is saying.
Not forgetting Torah
- There’s a Mishna (Pirkei Avot 3:8) that states that one who forgets Torah is responsible for his soul and violates the prohibition of “רק השמר לך ושמר נפשך מאד פן תשכח את הדברים אשר ראו עיניך” (Beware and take caution for your soul very much lest you forget the things which your eyes have seen). However, the Mishna continues that if one learned more than one was able to remember then there's no violation committed. Some understand the prohibition as including someone who was inadvertent and exempts only the true extenuating circumstances. However, others understand that the prohibition only applies to someone who's negligent and actively forgot Torah. 
- The foods that Chazal specify as causing one to forget Torah such as food from which a mouse or cat ate from are permissible to eat, however, it’s preferable not to eat them. However, there’s no issue for a women to eat it. 
- Even though the opinion of Rabbi Shimon Bar Yochai in Menachot 99b is that one can fulfill one's obligation of learning by just learning one piece of torah in the morning and one in the night, there is still a concept of Bitul Torah. The Rambam (Talmud Torah 1:8) and Shulchan Aruch YD 246:1 codify the opinion of Rabbi Shimon Bar Yochai, nonetheless it is clear that they also hold of the concept of Bitul Torah (see Bet Yosef YD 245:22) as will be detailed below.
- Menachot 99b, Nedarim 8a says that one exempts oneself from Talmud Torah daily with reading Kriyat Shema. However, Ran (Nedarim 8a) and Ritva (on Rif Nedarim 8a) say that Kriyat Shema isn’t an exemption except for someone who doesn’t have any free time because of business. Similarly, Tosfot (Brachot 11b s.v. Shekevar) writes that one doesn’t make new Torah Brachot the whole day because one’s mind is on Torah since one’s obligated in it all day. [Bet Yosef 47 uses the reason that one’s obligated all day to explain why there’s no bracha after Brachot HaTorah.] Yet, Sh”t Radvaz 3:416 and Rashba (Nedarim 8a) explain the gemara that literally one exempts himself with Shema. Chida in Machzik Bracha 156:1, Mishna Brurah 155:4, and Halacha Brurah 155:2 rule that if one has free time one has an obligation to learn and not doing so would be Bitul Torah.
- The commonly quoted Gemara that mentions the severity of the sin of Bitul Torah is Sanhedrin 99a which says that Bitul Torah is considered degrading Hashem’s word. Shabbat 32b says that one’s children die for Bitul Torah. Masechet Kala Rabati 6:4 says that Tzaddikim die because of Bitul Torah (of the generation). Midrash Rabba Eicha 1:20 says that Galut is caused by that sin. Tanit 7b says that for that sin it stops to rain. Someone who is able to learn and doesn't may get the punishment of a physical illness (Brachot 5a) and Hashem cries for him (Chagigah 5b).
- Rambam, Mishneh Torah: Hilchot Talmud Torah: Chapter 3, Halachah 4
- Shulchan Aruch O"C 231
- Berachot 5a
- Birurei Chayim v. 1 ch. 12 discusses this question at great length. He cites the Rabbi Akiva Eiger 47:12, Chikrei Lev OC 11a, Chazon Ish (Teshuvot Vketavim OC Zeraim Kodshim), Or Same'ach Talmud Torah 1:2 who say that Shema isn't learning when done as a mitzvah of Shema. See there for many more opinions. Regarding Tehillim, he cites Halichot Shlomo Tefillah p 105 n. 22 who says saying tehillim for tefillah is also learning. See there for many more opinions.
- Rabbenu Yonah on Pirkei Avot 3:8
- Rashbetz in Magen Avot
- Sh”t Yabia Omer Y”D 2:8(4). See also Shulchan Aruch HaRav Hilchot Talmud Torah 3:1 in the footnote.