Reciting Birchas Hatorah prior to a public Shiur:
It is forbidden to learn Torah prior to reciting Birchas Hatorah.[1] This applies even for an unlearned Jew.[2] This applies even if one will only be hearing the Torah words said by another and not verbalizing it himself.[3] Thus, it is proper for those who teach Torah classes to non-observant Jews to arrange prior to the start of the class for one of the participants to recite Birchas Hatorah aloud and fulfill the obligation for all the other participants. He should likewise say the verses of Yivarechicha, as is normally done, and not rely simply on the lesson being taught.[4] [However, the teacher himself should not say the blessing on their behalf.[5]] This applies to both classes attended by men or women.[6] This applies whether the class is taking place by day or by night.[7] Aside for fulfilling the Mitzvah and obligation of Birchas Hatorah through doing so, it is also emphasizes to the participants that they are not learning any random wisdom, but the wisdom of G-d.
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[1] Admur 47:2; Siddur Admur; Ketzos Hashulchan 5:8; and so is explicitly implied from Shulchan Aruch 47:1 [implies one may never learn Torah before the blessing-M”B 47:2] 47:2 [implies that it is forbidden to verbalize the words beforehand, and not just that one may say the blessing if he verbalizes the words], 46:8. This is simply to negate those that understood that the prohibition of learning Torah prior the blessing is a novelty of the Siddur. Upashut! So rules also: Bitzel Hachochma 1:1; Yosef Ometz 66; Chayeh Adam 9:11; Meishiv Davar 47
[2] See Hilchos Talmud Torah 2:12
[3] Halachos Ketanos 2:159; Tehila Ledavid 47:2; Ben Ish Chaiy Vayeishev 12; Keren Ledavid 11; Erech Hashulchan 47:2; Shaareiy Teshuvah 47:3; Kaf Hachaim 47:5
Other opinions: Some Poskim rule one may listen to a person saying words of Torah prior to reciting Birchas Hatorah. [Machazik Bracha 47:4; Zekan Ahron 60; Halef Lecha Shlomo 35; See Sdei Chemed Asifas Dinim Mareches Rosh Hashanah 2:19]
Opinion of Admur: Admur states that one does not say a blessing when thinking Torah or even saying a Halachic ruling, being that it is like a mere thought. The reason that thought does not require a blessing is because it is not like speech. [47:2] In Hilchos Talmud Torah 2:12 Admur states that one who hears words of Torah from another fulfills the Mitzvah of learning Torah as hearing from another is like speech. This would hence imply that a blessing is recited. Nevertheless, perhaps Admur requires actual speech, just like he writes in 104:5 that one may listen to Kedusha during Shemoneh Esrei, as although listening is like speech, since it is not verbalized it is not an interval. Hence, we see that it is not like speech for all matters. Practically from the simple understanding of 47:2 it would appear that according to Admur one must say a blessing. Vetzaruch Iyun.
[4] As perhaps according to the Poskim who rule that hearing words of Torah is permitted before Birchas Hatorah would also rule that one does not fulfill the Mitzvah of Talmud Torah in this regard, if he simply hears Torha, and hence it is proper to say the actual verses.
