OPINIONS: Rebbi Yochanan states that a Talmid Chacham who walks in public with a "Revav" (stain) on his clothing is Chayav Misah. Ravina states that he may not go out with a "Revad." The Gemara explains that they do not argue; one refers to the Talmid Chacham's overgarment, and the other refers to the shirt underneath his overgarment.
What is the difference between a "Revav" and "Revad," and what is the difference between a stain found on the overgarment and a stain found on the shirt beneath the overgarment?
(a) RASHI says that a "Revav" is a stain of oil or fat. Oil or fat is forbidden only on one's overgarment, where it can be seen by all.
"Revad" is a stain of Shichvas Zera. A stain of "Revad" is forbidden even on one's undergarment, because it gives the impression of Shichvas Zera and is exceedingly disgraceful.
(b) RABEINU CHANANEL says that "Revav" refers to a stain of tar. The reason a Talmid Chacham may not walk out with a tar-stain on his garment is because it causes people to stay away from him in fear that the tar on his clothing might touch and stain their clothes. Tar is forbidden even on one's shirt beneath the overgarment for this reason.
"Revad" refers to blood. A Talmid Chacham may not go out with a bloodstain because it looks like a "Kesem" (a bloodstain from a woman who was a Nidah). People might think that it is from a woman and they will stay away from him, because they are disgusted by the blood. However, a bloodstain is forbidden only when it is on the overgarment, because when it is on the undergarment, everyone will assume that it came from a bug that he killed (or from a scratch on his body), and they will not think it is a "Kesem" and stay away from him.


QUESTION: Rav Shisha maintains that the Shofar is blown on Shabbos or a festival to indicate that something that is forbidden today will become permitted tomorrow. The Gemara says that according to Rav Shisha, when Yom Tov occurs on Motza'i Shabbos the Shofar should be blown right after Shabbos to indicate that it is now permitted to slaughter an animal.
Why does the Gemara mention specifically Motza'i Shabbos which coincides with Yom Tov? The Gemara should ask that the Shofar should be blown every Motza'i Shabbos to indicate that Melachah is no longer forbidden. (RITVA)
ANSWER: The RITVA answers that the Gemara knows that there is no reason to blow the Shofar simply to indicate that things that were forbidden until now have become permitted. Rather, the Gemara assumes that if something now becomes permitted which is also a Mitzvah to do, then the Shofar should be blown. On Yom Tov, slaughtering an animal is not only permitted, but it is a Mitzvah as well (for the sake of Simchas Yom Tov).
QUESTION: On a regular Yom Kippur, one is permitted to clip the leaves ("Kenivas Yerek") off of a [detached] vegetable towards the evening in preparation for the meal for that night. Rav Huna says that when Yom Kippur occurs on Shabbos, one may not clip the leaves off of a vegetable. The Gemara cites a Beraisa which seems to support this ruling from a verse in the Torah.
Rebbi Yochanan, however, says that one is permitted to clip the leaves of a vegetable on Yom Kippur that occurs on Shabbos. The Gemara attempts to refute Rebbi Yochanan's opinion from the Beraisa. The Gemara answers that when the Beraisa says that "Kenivas Yerek" is forbidden on Yom Kippur that falls on Shabbos, it does not refer to clipping the leaves off of a detached vegetable, but it refers to a Melachah d'Oraisa of cutting the vegetable from the ground (the Melachah of Kotzer). The Gemara explains that even though there is already a negative prohibition (Lo Ta'aseh) that forbids Melachah, the Beraisa teaches that there is also a Mitzvas Aseh that commands us to refrain from doing Melachah on Shabbos.
According to the Gemara's answer, what does this Mitzvas Aseh have to do with Yom Kippur that falls on Shabbos? The prohibition, and Mitzvas Aseh, against doing Melachah apply every Shabbos!
In addition, if the Beraisa derives from the verse that there is a Mitzvas Aseh to refrain from Melachah, why does the Beraisa specify this particular Melachah of cutting a vegetable from the ground?
(a) The BA'AL HA'ME'OR and the RAMBAN explain that the Gemara is not explaining the Beraisa. The Beraisa indeed contradicts the opinion of Rebbi Yochanan. (There is another Beraisa which supports Rebbi Yochanan, as the Gemara says later). Rather, the Gemara here is merely answering how Rebbi Yochanan learns the verse that the Beraisa cites, so that the verse itself will not be a contradiction to Rebbi Yochanan's opinion. Rebbi Yochanan learns from this verse the Isur Aseh against performing any Melachah on Shabbos.
(b) TOSFOS (DH l'Olam l'Melachah) understands that the Gemara is indeed explaining the Beraisa when it says that the "Kenivas Yerek" mentioned in the Beraisa is a Melachah d'Oraisa (see RAV ELAZAR MOSHE HA'LEVI HOROWITZ). How, then, does Tosfos understand the Beraisa? Why does the Beraisa discuss Yom Kippur that falls on Shabbos, and why does it mention in particular the Melachah of Kenivas Yerek?
Perhaps Tosfos understands the Gemara like the TOSFOS HA'ROSH. The Tosfos ha'Rosh explains that when the Gemara says that the Beraisa derives from the verse that Melachah is also forbidden due to a Mitzvas Aseh as well as a Lo Ta'aseh, it does not mean that there is actually a Mitzvas Aseh d'Oraisa to refrain from Melachah. Rather, the verse is merely an Asmachta, a textual support for a rabbinical enactment. The Rabanan wanted to teach that when one clips the leaves off of a detached vegetable on Yom Kippur that occurs on Shabbos, he should be careful to clip only the leaves, and not to clip small pieces of the vegetable, for that would constitute an Isur d'Oraisa (Tochen). The Rabanan relate this warning to us in the form of a verse from the Torah (as though their warning is contained in the Torah). When the Rabanan in the Beraisa cite the verse in the Torah, they are teaching that it is forbidden to clip the leaves off of a vegetable in a way that is forbidden to do on Shabbos.