1) HALACHAH: REBUKING A WRONGDOER
The Gemara teaches that when one sees another person commit a transgression without knowing that the act is prohibited, he should not rebuke the wrongdoer if he knows that the wrongdoer will continue to sin ("Mutav she'Yiheyu Shogegin v'Al Yiheyu Mezidin"). The Gemara says that this principle applies only to transgressions of rabbinical enactments, but not to transgressions of Torah prohibitions. The Gemara then says, "This is not so; rather, there is no difference between a rabbinical prohibition and a Torah prohibition" (and in both cases one should not rebuke the wrongdoer).
The Rishonim (see RAN and ROSH in the name of the BA'AL HA'ITUR) explain that when the Gemara says that one is not required to rebuke a person who unintentionally commits a Torah prohibition, it refers only to a prohibition that is not written explicitly in the Torah (but rather is derived through Halachah l'Moshe mi'Sinai or through one of the ways in which Torah laws are derived).
The Poskim (OC 608:2) discuss the practical applications of this Halachah in the context of the general requirement to rebuke a wrongdoer.
(a) One must rebuke a person who willfully sins, even if he feels certain that the sinner will not heed the rebuke. If the wrongdoer is transgressing a prohibition written explicitly in the Torah, it is assumed that he knows that it is forbidden and that he is sinning intentionally, and therefore one must rebuke him and tell him to stop.
(b) If one sees that the intentional wrongdoer does not accept the rebuke, then he should continue to rebuke the wrongdoer until the wrongdoer gets upset and yells at him. However, this rebuke should be administered in private and not in public. (One rebukes a sinner in public only once.) Also, if the sin was committed in private one should rebuke the sinner only in private.
The BI'UR HALACHAH (OC 608, DH Chayav) writes in the name of the SEFER CHASIDIM (#413) that one is not required to rebuke a wrongdoer until the wrongdoer yells at him unless he feels close to the person (that is, they know each other well and there is a chance that the rebuke will be accepted).
(It is important to remember that the purpose of the Mitzvah of rebuke is exclusively to help the wrongdoer correct his ways and follow the will of Hash-m and His Mitzvos. Rebuke is not for the sake of deriding, insulting, or punishing the wrongdoer. In order to be effective, rebuke must be administered in different ways, depending on the level of scholarship and maturity of the person who sinned, as well as on the relationship between the wrongdoer and the one who gives the rebuke.)
(c) When the wrongdoer's sinful act is forbidden mid'Oraisa but is not stated explicitly in the Torah, or it is prohibited only mid'Rabanan (and it is not evident from his actions that he knows that the act is prohibited), if one knows for certain that the wrongdoer will not heed the rebuke he is not required to tell the wrongdoer to stop. In such a case, the rebuke will only make his sin worse by changing it from an unintentional sin to an intentional one, as the Gemara here teaches.
If there is a possibility that the offender will listen to the rebuke, then one is obligated to rebuke him (as the Gemara says in Shabbos 55a). (See also Insights to Shabbos 55:1 and 148:2.)
2) A "TENAI" TO PERMIT MUKTZAH
QUESTION: The Gemara quotes a Beraisa which discusses various laws of Muktzah. The Beraisa says about the wood of a Sukah that if one makes a condition (Tenai) and expresses his intent to use the wood of the Sukah on Yom Tov, then the Tenai works to permit him to use the wood. However, the Tenai works only for the wood of a Sukah on a Yom Tov other than Sukos. On Sukos, a Tenai does not work to permit the use of the wood of a Sukah Shel Mitzvah because the Torah gives Kedushah to such a Sukah at the moment Yom Tov enters. As soon as Bein ha'Shemashos arrives the Sukah becomes designated for use as a Sukah and its wood may not be used for any other purpose.
A Tenai works to permit the use of the wood of a Sukah during the rest of the year (provided that the Sukah is "Re'u'ah," unsteady and there is a likelihood that it will fall). One may make a Tenai and say, "I intend to use the wood of the Sukah during Yom Tov. Even though the wood is part of the Sukah now and it cannot be dismantled on Yom Tov, I intend to use it in the event that the Sukah collapses on Yom Tov" (and thus using the wood involves no prohibition of Stiras Ohel, destroying a structure; RASHI DH Seifa and DH Abaye). The RAMBAN (in Milchamos) extends the application of this Halachah to the wood of a Sukah Shel Mitzvah on Sukos. He says that one may make a Tenai to use the wood of such a Sukah in the event that the Sukah collapses.
The Gemara's allowance to make a Tenai is problematic. The Gemara here quotes a Beraisa (from Shabbos 44a) which states that the oil of an oil-lamp is Muktzah according to Rebbi Yehudah even though the person waits ("Yoshev u'Metzapeh") for the candle to go out so that he can use the oil. "Yoshev u'Metzapeh" seems to be no different than a Tenai, and yet the object remains Muktzah. Why, then, in the case of the wood of a Sukah does one's Tenai permit the wood in the event that the Sukah collapses? Since the wood was Muktzah Machmas Isur during Bein ha'Shemashos, it should remain Muktzah just like the candle, and a Tenai should not work. (TOSFOS to Shabbos 44a, DH sheba'Ner)
(a) TOSFOS suggests that a Tenai does not work in the case of the oil of an oil-lamp because when one lit the lamp he actively and purposefully removed it from use on Shabbos or Yom Tov ("Docheh b'Yadayim"). By lighting it, he showed that he wanted it to be in a state of Isur (giving off the light of a flame) on Shabbos. In contrast, one has no need for a Sukah to be standing on Yom Tov (other than Sukos) and it is not considered to be actively and purposefully removed from use. Therefore, a Tenai works.
(b) The TOSFOS YESHANIM in Shabbos answers that a lamp remains prohibited because it falls into two categories of Muktzah: it is Muktzah Machmas Isur, and it is a "Basis l'Davar he'Asur" because it supports the flame which is a "Davar he'Asur" (Shabbos 47b). The wood of a Sukah, in contrast, is prohibited only because it is Muktzah Machmas Isur, and therefore a Tenai works.
(c) The RAMBAN in Shabbos (45a) and the RASHBA, RE'AH, and RAN here quote the Yerushalmi (Shabbos 3:7) which implies that a Tenai works even in the case of an oil-lamp. When the Gemara says that the oil of a lamp may not be used on Shabbos even after the flame goes out, it refers to when one was only "Yoshev u'Metzapeh" in anticipation of the flame going out but he did not explicitly state a Tenai. If he explicitly states a Tenai, then his Tenai works to permit the oil.
(d) The BA'AL HA'ME'OR explains that when the Gemara permits one who made a Tenai to use wood from a Sukah, it does not refer to wood which, at the onset of Yom Tov, was prohibited to use because of the Melachah of Stirah mid'Oraisa. Rather, the wood for which a Tenai works is wood that, when removed, does not cause the Sukah to be dismantled. Even though the wood is subordinate (Batel) to the Sukah, since the Sukah remains standing when one takes the wood his act is not considered an act of demolishing (Stirah) but it merely looks like Stirah and is prohibited mid'Rabanan. Since the wood is Muktzah only because the act looks like an act of Stirah when one removes the wood on Yom Tov, the Tenai is effective.
In contrast, in the case of an oil-lamp it is the Torah prohibition of Mechabeh (extinguishing) which prohibits the candle from being extinguished during Bein ha'Shemashos. Therefore, a Tenai does not work to remove its status of Muktzah.
(e) The RA'AVAD quotes the Yerushalmi which gives a different explanation for the Beraisa cited by the Gemara. When the Beraisa says that a Tenai works, it does not refer to the wood that was used to build a Sukah. Rather, it refers to wood that was tied up in a bundle and placed on top of the Sukah. The Yerushalmi says that to remove such wood is prohibited because of Stirah unless one made a Tenai. (The Gemara here, in contrast, permits the removal of such bundles even without a Tenai because the act does not constitute Stirah.) The Tenai says that the person does not want the bundle of wood to become Batel to the Sukah. Without a Tenai, the wood becomes Batel to the Sukah and is therefore prohibited to use because of Stirah.
According to the Ra'avad's explanation, why does the Gemara here explain that the Beraisa, which says that a Tenai works, refers specifically to the wood of an ordinary Sukah? According to the Ra'avad's explanation, it does not matter what type of Sukah it is as long as the wood is not built into the Sukah but is tied in a bundle and placed on top of the Sukah.
The Ra'avad answers that the text of the Gemara here which says that the Beraisa refers to an ordinary Sukah is a mistake and does not belong in the Gemara. Alternatively, the Ra'avad explains that the Gemara disagrees with the Yerushalmi only in its initial assumption (Havah Amina). Once the Gemara mentions the decorations of a Sukah and says that the allowance to make a Tenai works for the decorations of a Sukah Shel Mitzvah and prevents them from acquiring the Kedushah of the Sukah, it no longer maintains that a Tenai works to permit something which is Muktzah as a result of the prohibition of Stirah, in agreement with the Yerushalmi.
The Ra'avad says that this is the opinion of the RIF. This is also implied by TOSFOS (DH Amar Rav Menasheh, as the VILNA GA'ON mentions in his notes to the Gemara) and by the RAMBAM (Hilchos Sukah 6:15), as the BEIS YOSEF writes (OC 518).
HALACHAH: The SHULCHAN ARUCH (OC 279:4) rules that a Tenai works even to permit the use of oil in an oil-lamp, as the Ramban and other Rishonim (in (c) above) rule. The BI'UR HALACHAH (OC 518, DH Notlin) writes that even according to the Shulchan Aruch's ruling, a Tenai helps only when the item is expected to become permissible later ("Yoshev u'Metzapeh"); a Tenai does not work to permit the use of wood of a Sukah which is strong and is not expected to collapse.
The REMA (OC 279:4) is stringent and writes that one should not rely on the Ramban to permit Muktzah Machmas Isur with a Tenai unless there are other grounds to be lenient (for example, a Tenai works to permit a Nochri to handle the item).