1) THE MELACHAH OF "KOTESH" (POUNDING)
QUESTIONS: RASHI (DH she'Ken Ani) explains that the Melachah of Kotesh, pounding, was done in the Mishkan and it is an Av Melachah. However, it is not counted in the Mishnah, because the Mishnah counts only labors involved in "Sidura d'Pas" (making bread), and pounding is not considered an established part of Sidura d'Pas because poor people do not do it when they make their bread. Nevertheless, "it is an Av Melachah, included in the Melachah of Dash (threshing)."
(a) Rashi contradicts himself. He first writes that Kotesh is an Av Melachah, and then he writes that it is included in Dash, clearly implying that it is a Toldah of Dash!
(b) Rashi implies that there are forty Av Melachos. How can this be, when the Mishnah clearly states that there are no more than thirty-nine Melachos? (The Gemara (75b) proves that there are only thirty-nine Melachos according to the Mishnah.)
(c) Why does Rashi need to mention "Sidura d'Pas" at all? The Gemara says that a poor person does not pound his bread, and therefore Kotesh is not in the Mishnah either because it is not considered a significant labor, or because it was not done in the Mishkan. Why does Rashi have to mention that it is not part of "Sidura d'Pas"? (MAHARSHA, MAHARAM)
(d) Why does Rashi need to say that Kotesh is an Av Melachah but is not counted in the Mishnah? He should have said that since a poor person does not do Ketishah, it is not an Av Melachah altogether!
ANSWER: The RAMBAN writes in the name of Rashi that Kotesh is not considered a significant labor since a poor person does not do it, and it is included in Dash (as we suggested in (d) above). Perhaps this is what Rashi intends to say, even in the text of our version of Rashi. When Rashi says that Kotesh is an Av Melachah, he means only that it is in the category of forbidden Melachah on Shabbos for which one will be Chayav, but not that it is its own primary category of Melachah. (This is consistent with Rashi's opinion elsewhere. It is evident in numerous places that Rashi uses the term "Av Melachah" to refer not to a primary category, but to any form of labor for which one is Chayav on Shabbos, including a Toldah. See Insights to Shabbos 18:1.) This answer questions (a), (b), and (d).
Concerning the third question (c), why Rashi mentions "Sidura d'Pas," it must be that Rashi understands the Gemara like the Ramban: since a poor person does not do Ketishah, it is not considered a significant Melachah. Rashi, however, was bothered by Tosfos' question. Tosfos asks that Ketishah was performed in the Mishkan for the preparation of the dyes, and not merely to make bread. Why is it not considered an Av due to the fact that it was needed for dye-production? Tosfos answers that a poor person even makes his dyes without pounding them. Rashi was bothered why the Gemara specifically mentions the bread of a poor person, and not his dyes.
To answer that question, Rashi, says that since all of the Melachos listed in the first part of the Mishnah refer to "Sidura d'Pas," and the Gemara is now discussing Kotesh in the context of Borer and other bread-related Melachos, it must be that the Kotesh under discussion is that which is used in "Sidura d'Pas." "Sidura d'Pas" has nothing to do with the reason why Kotesh is not mentioned in the Mishnah. Rashi is merely explaining why the Gemara focuses on bread ("since a poor person eats his bread without Ketishah") and does not mention the dyes. (M. KORNFELD; see SEFAS EMES for a different explanation.)
2) HALACHAH: THE MELACHAH OF "BORER"
OPINIONS: The Gemara cites a Beraisa that first states that Borer is permitted on Shabbos and then states that Borer is forbidden and one is Chayav to bring a Chatas for it. Five different resolutions are suggested in the Gemara to explain what the Beraisa means when it says that it is permitted to do Borer:
(a) It is permitted when done for use on that day.
(b) It is permitted when only half the Shi'ur is separated.
(c) It is permitted when done by hand ("b'Yad").
(d) It is permitted when one separates a food item from a non-edible item ("Ochel mi'Toch Pesoles").
(e) It is permitted when done for immediate use ("l'Altar").
The Gemara unconditionally rejects the first two answers. Doing Borer for that day is forbidden and one is Chayav a Chatas, and "Chatzi Shi'ur" is also forbidden mid'Oraisa (although one is not Chayav a Chatas). Which of the other answers are accepted? What is the Halachah?
(a) TOSFOS (DH v'ha'Tanya) and the ROSH cite RABEINU CHANANEL, who says that all three of the latter conditions must be fulfilled in order for it to be permissible to do Borer on Shabbos (or more exactly, in order for this act not to be one of Borer).
(b) RASHI (DH v'ha'Tanya) appears to say that if it is done immediately, it is permissible, even when done with an instrument made for separating (such as Napah u'Kevarah).
HALACHAH: The Halachah follows the opinion of Rabeinu Chananel, as the SHULCHAN ARUCH (OC 319) writes, and three conditions are required -- b'Yad, l'Altar, and Ochel mi'Toch Pesoles.
The REMA cites Rabeinu Chananel and explains that "l'Altar" refers to any food that is prepared for the meal that is about to come (even if it is not going to be consumed immediately).