1) HALACHAH: A "CHAZAKAH" THAT A "SHALI'ACH" DOES HIS JOB
OPINIONS: The Gemara discusses the principle that one may assume that a Shali'ach carried out his assignment -- "Chazakah Shali'ach Oseh Shelichuso." Rav Nachman maintains that this principle applies only to laws that are mid'Rabanan, and Rav Sheshes maintains that it applies even to laws that are mid'Oraisa. What is the Halachah?
(a) TOSFOS (DH Rav Sheshes) cites RABEINU SHIMSHON of Folierre (Falaise) who rules leniently, like Rav Sheshes, based on the BEHAG's rule that "the Halachah is in accordance with Rav Nachman with regard to monetary matters and like Rav Sheshes with regard to matters of prohibitions." This is also the conclusion of the RI and the ROSH.
(b) RABEINU TAM (cited by Tosfos) rules like Rav Nachman, who says that this Chazakah is only reliable with regard to laws that are mid'Rabanan, because there are several Sugyos throughout Shas that seem to favor his opinion. This is also the opinion of the RIF and RAMBAM. (See also MISHNEH L'MELECH, Hilchos Bechoros 4:1, who concludes that one should be stringent and follow this opinion.)
2) USING A TREE ON SHABBOS BY PLACING AN "ERUV" IN IT
QUESTION: The Mishnah states that when one places the food of his Eruv in a tree within ten Tefachim from the ground, the Eruv is valid, because he and his Eruv are in the same domain (Reshus ha'Rabim), and therefore the Eruv is accessible on Shabbos.
The Gemara suggests that even if an Eruv is placed high up in the tree, if the person and his Eruv are in the same domain (Reshus ha'Yachid), then the Eruv is valid. The Gemara challenges this assertion and says that the Eruv should not be valid, because when one places the Eruv in the tree, he uses the tree ("Mishtamesh b'Ilan"), and the Rabanan prohibited the use of a tree on Shabbos.
What "use" of the tree occurs when one places his Eruv in it? He does not need to climb the tree to get the Eruv, since it is only ten Tefachim high. One is not prohibited to remove the Eruv from the tree, because there is no use of the tree involved in such an act. Indeed, the Gemara in Shabbos (153a) clearly states that one is permitted to remove items from the back of an animal, even though the use of an animal, like the use of a tree, is prohibited. The fact that the tree holds something that one placed there before Shabbos also cannot be considered use of the tree, because the Gemara in Shabbos (45a) says that one may place a candle in a tree to remain there during Shabbos. (RITVA, RASHBA)
(a) The RITVA cites one opinion that says that even to derive a Halachic benefit from a tree (i.e., his Eruv in the tree makes the tree his primary place of Shabbos dwelling and thereby alters his Techum Shabbos) is considered "Mishtamesh b'Ilan."
The Ritva rejects this answer. One cannot compare Halachic benefit derived from the tree with use of the tree, because the decree against use of a tree on Shabbos was enacted in order to prevent one from accidentally cutting down a branch. There is no concern that one might cut down a branch when he derives Halachic benefit from the tree.
(b) The Ritva explains that the Rabanan were concerned that one might lean on the tree while he removes the food of his Eruv.
Based on this Gemara, the REMA (OC 336:1) rules that one may not remove any object from a tree on Shabbos, because one might lean on the tree (Mishnah Berurah 336:12, Sha'ar ha'Tziyun 336:6-7).
3) DO THE PROHIBITIONS OF "SHEVUS" APPLY DURING "BEIN HA'SHEMASHOS"?
QUESTION: The Gemara quotes Rebbi who rules that the prohibitions of "Shevus," or the Isurei d'Rabanan of Shabbos, do not apply during Bein ha'Shemashos. Rebbi's ruling is recorded as the Halachah (SHULCHAN ARUCH OC 307:22, 342:1).
If the Gezeiros d'Rabanan do not apply during Bein ha'Shemashos, then why does the Mishnah in Shabbos (34a) state that it is prohibited to immerse utensils in a Mikvah or separate Ma'aser during Bein ha'Shemashos? Those acts are prohibited on Shabbos only mid'Rabanan. Does Rebbi argue with that Mishnah and permit those acts to be performed during Bein ha'Shemashos?
(a) The RITVA cites the RA'AVAD who explains that Rebbi certainly agrees with the Mishnah in Shabbos. Rebbi's ruling that the prohibitions of "Shevus" do not apply during Bein ha'Shemashos refers only to the laws of Eruv Techumin. Rebbi states that when the Rabanan enacted the laws of Eruvei Techumin, they declared that an otherwise valid Eruv is valid as long as there is no Isur d'Oraisa which prevents the person access to the food of his Eruv. However, if there is an Isur d'Rabanan that prevents access to the Eruv, the Eruv remains valid. The Rabanan never actually permitted one to transgress an Isur d'Rabanan during Bein ha'Shemashos in order to get to the Eruv. (This is similar to the concept that Rashi mentions later (end of 32b) when he says that the area of a person's Eruv Techumin is considered like a Reshus ha'Yachid that extends up to the sky, even though it is not really a Reshus ha'Yachid.)
(b) The RAMBAM (Hilchos Shabbos 24:10) writes that one is permitted to transgress an Isur of "Shevus" during Bein ha'Shemashos for the sake of a Mitzvah. Since an Eruv is made for the sake of a Mitzvah (such as to learn Torah from a teacher who is farther than 2000 Amos away), one is permitted to do an Isur of "Shevus" for that purpose. Rebbi agrees, however, that one may not separate Ma'aser from fruit during Bein ha'Shemashos, since one does not separate for the sake of a Mitzvah. (MAGID MISHNEH ibid.)
(c) RASHI in Beitzah (19a, DH Meida Yada, according to the MAHARSHA there and SHA'AR HA'MELECH, Hilchos Eruvin 1:22) explains that even during Bein ha'Shemashos, one may not perform an act that is not needed for Shabbos at all. (For this reason, one is prohibited to immerse a utensil in a Mikvah during Bein ha'Shemashos in order to purify the utensil for use with Terumah. Since the utensil will become permitted to use for Terumah only after sunset following Shabbos (at which time the utensil becomes fully Tahor), the Tevilah is not an act that is needed for Shabbos at all.)
Accordingly, this might be the type of Tevilah that the Mishnah in Shabbos (34a) prohibits. The same rule applies to Ma'aser during Bein ha'Shemashos. If one will not eat the food on Shabbos (for example, it is raw at the moment, and it is normally eaten cooked), one may not separate Ma'aser from it during Bein ha'Shemashos. (See also Insights to Beitzah 19:1.)