Gemara

Question: Why don't Chachamim usually descend from Chachamim?

Answer (Ravina): It is because they do not bless before learning Torah;

(Rav Yehudah): "Who is the Chacham who knows this (why Yisrael were exiled)?" Chachamim and Nevi'im could not explain it, until Hash-m explained it - "because they abandoned My Torah".

Question: "They did not listen to My voice" is the same as "they did not go in My way"!

Answer (Rav Yehudah): "They did not go in My way" means that they did not bless before learning.

Berachos 11b - Question: What is the text of Birkas ha'Torah?

Answer #1 (Rav Yehudah): It is "Asher Kideshanu b'Mitzvosav v'Tzivanu La'asok b'Divrei Sorah."

R. Yochanan would conclude "Ha'arev Na... Divrei Sorascha b'Finu... Baruch Atah Hash-m ha'Melamed Torah l'Amo Yisrael."

Answer #2 (Rav Hamnuna): It is "Asher Bochar Banu mi'Kol ha'Amim V'Nosan Lanu Es Toraso. Baruch Atah Hash-m Nosen ha'Torah."

(Rav Hamnuna): This is the greatest Berachah.

Therefore, one should say all of them.

21a - Question: What is the source that mid'Oraisa, we must bless before learning Torah?

Answer: "Ki Shem Hash-m Ekra Havu Godel l'Eilokeinu."

(R. Yochanan): A Kal va'Chomer obligates blessing before eating mid'Oraisa! We bless before learning Torah, but not afterwards. We bless after eating, all the more so we must bless before!

Objection #1: We cannot learn from Torah, for Torah gives one eternal life!

Objection #2 (Mishnah): A Ba'al Keri (one who had a seminal emission) blesses after eating, but not before eating (for the latter is only mid'Rabanan).

R. Yochanan is refuted.

Berachos 48b (Beraisa): Question: What is the source for Birkas ha'Torah?

Answer #1 (R. Yishmael): If we must bless for (bread, which gives) temporary life, all the more so for Torah, which gives eternal life!

Answer #2 (R. Chiya bar Nachmani citing R. Yishmael): We do not need the Kal va'Chomer. It says "Al ha'Aretz ha'Tovah Asher Nosan Lach," like it says "v'Etnah Lecha... veha'Torah."

Rishonim

Rif (Berachos 5b): We say all of them (the three texts of the Amora'im).

Rambam (Hilchos Tefilah 7:10): One blesses three Berachos: Asher Kideshanu b'Mitzvosav... Ha'arev Na... and Asher Bochar Banu...

Lechem Mishneh: The Rambam's text did not say 'R. Yochanan concluded...'. Therefore, he explains that they are three Berachos.

Rosh (Berachos 1:13): R. Tam says that a short Berachah must begin with Baruch, even if it follows another Berachah. If so, "Ha'arev Na... " is the conclusion of the first Berachah, and we say "v'Ha'arev Na..." Birkas ha'Torah has only two Berachos; the text says 'therefore, we say both of them.'

R. Yonah (cited in Ran Nedarim 81a DH Davar): The verse teaches that they did not bless on the Torah. We cannot explain literally that they were not learning, for then Chachamim and Nevi'im would have known that this caused the Churban! Hash-m, who knows what is in the heart, explained that it was because Torah was not important enough in their eyes to consider it worthy of a Berachah. They did not engage Lishmah. Therefore, they disgraced its Berachah.

Sha'agas Aryeh (24): Yisrael would not have been worthy of destruction for flattering Agripas if mid'Oraisa he was Kosher for kingship (Tosfos Sotah 41b DH Oso). Likewise, if Birkas ha'Torah were only mid'Rabanan, disgracing it would not cause a Churban that one's children will not be Chachamim.

Poskim

Shulchan Aruch (OC 47:1): One must be very careful about Birkas ha'Torah.

Beis Yosef (DH Birkas): Rashi explained that amidst Chachamim's zeal to engage in Torah, they are not careful about the Berachah. Therefore it was not fulfilled for them 'our descendants will engage in Torah.' Mahari Avuhav says that not blessing shows that they do not engage in it Lishmah, rather, like a mere job. Therefore, they do not merit "Lo Yamushu mi'Picha..."

Bach (DH u'Mah she'Chosav d'Omar): Hash-m gave us Toras Emes in order that our Neshamos and bodies will cling to the Mitzvos, and the Shechinah will dwell amidst us. Yisrael did not do so. The learned only in order to know laws needed for business, and to flaunt their Chachmah. This caused the Shechinah to depart. The Berachah Asher Bochar Banu alludes to that Hash-m brought us to Har Sinai and gave us His cherished Torah in order that we will cling to its Kedushah.

Taz (1): To reach the Halachah, one must give and take and toil in Torah. They did not bless because they were not exerting themselves in Torah.

Mishnah Berurah (1): Sha'agas Aryeh (24) concludes that Birkas ha'Torah is mid'Oraisa. Therefore, if one is unsure whether or not he blessed, he must bless again. However, he blesses only Asher Bochar Banu, the greatest of the Berachos. Some Acharonim say to be passive and not bless mi'Safek. One should not rely on this. Many Rishonim say that it is mid'Oraisa; the punishment for neglecting it is great. However, if one remembered after Shacharis that he did not bless, he should not bless now, for he blessed Ahavah Rabah. Perhaps this is even if he did not learn immediately after praying. Since many say that Birkas ha'Torah is mid'Rabanan, he should ask someone else to exempt him, or to intend that Ahavah Rabah be Birkas ha'Torah and learn right after praying. All agree that Birkas ha'Torah for Kri'as ha'Torah in a Tzibur is mid'Rabanan, for one already blessed in the morning.

Kaf ha'Chayim (2): The Rambam and Tur did not specify; this implies that Birkas ha'Torah is mid'Rabanan. It seems that the Shulchan Aruch agrees, for in Siman 209 (3) it says that only Birkas ha'Mazon is mid'Oraisa. If one is in Safek and cannot be Yotzei from someone else, he should say it without Shem and Malchus, and have intent the next time he says Ahavas Olam. Even the Sha'agas Aryeh holds that only one Berachah is mid'Oraisa, so if one is sure that he said one of the Berachos, he does not repeat anything.

Note: R. Yochanan derived blessing (mid'Oraisa) before eating from a Kal va'Chomer from Birkas ha'Torah. We do not learn Divrei Torah from Divrei Sofrim (Yadayim 3:2)! The Gemara did not refute him from this! R. Yishmael learned Birkas ha'Torah from Birkas ha'Mazon. R. Chiya disagreed because a verse teaches this. This shows that he agrees that it is mid'Oraisa!

Mishnah Berurah (2): One must be very careful not to learn before blessing, and to bless with great Simchah. As long as Yisrael learned Torah, it protected them. When they did not bless because they considered Torah like other Chachmos, it did not protect. Therefore one must thank Hash-m for choosing us and giving us His cherished Keli. Also, one who is not careful about Birkas ha'Torah does not merit to have a son who is a Chacham.

Kaf ha'Chayim (1): The carefulness is to say it with understanding, gently, one word at a time. He should passionately feel that Torah is his life in this world and the next.