1) DERIVING A HALACHAH FROM A "MEH HA'TZAD"
QUESTION: The Gemara says that Rebbi Eliezer needs a separate verse to teach that the preparations for each of various Mitzvos are permitted to be done on Shabbos, even though they involve a Melachah. The Gemara explains why one verse does not suffice to serve as the source to permit the preparations for all Mitzvos on Shabbos.
Normally, when two verses teach that a certain law applies to two different categories of Halachah, we derive that it applies to all other categories through the method of "Meh ha'Tzad." Why, then, does the Gemara not suggest that we derive Rebbi Eliezer's law (that the preparations of a Mitzvah override Shabbos) from two verses through a Meh ha'Tzad?
(a) TOSFOS (DH Iy m'Omer), cites RABEINU PORAS who answers that the Gemara must have known that there was some question (Pircha) that prevented the Halachah from being derived through a Meh ha'Tzad. Had the Gemara suggested a Meh ha'Tzad, it would have countered that each of the cases mentioned has some unique feature that distinguishes it from all other cases, rendering it invalid as a source for a Meh ha'Tzad.
(b) Tosfos suggests a second answer. It is true that he Gemara could have derived Rebbi Eliezer's law from a Meh ha'Tzad. Nevertheless, it is possible that the Torah writes an additional verse to teach a law that could have been deduced through a Meh ha'Tzad in order to make it more clear. (This is known as "Tarach v'Kasav Lah Kra"; the Gemara itself actually applies this principle only to what is learned from a Kal v'Chomer; see Pesachim 18b.) Therefore, the Gemara does not find it necessary to show that the Halachos in question could not have been learned from a Meh ha'Tzad.
However, if the Halachah could have been derived from one verse through a Binyan Av, there would have been no need for the verse to mention it explicitly. A Binyan Av is just as clear as an explicit teaching. (See end of Insights to Chulin 117:2.)
2) DERIVING A HALACHAH FROM SUKAH TO LULAV
QUESTION: The Gemara says that according to Rebbi Eliezer, the Halachah that preparations for the Mitzvah of Lulav override Shabbos is derived from the word "ba'Yom" which appears in a verse that discusses the Mitzvah of Lulav. The Rabanan who argue with Rebbi Eliezer maintain that the phrase "ba'Yom" is needed to teach that the Mitzvah of Lulav is performed only during the day and not at night; we might have compared Lulav to Sukah through a Gezeirah Shavah (from the words "Shiv'as Yamim") and assumed that just as the Mitzvah of Sukah applies at night, so, too, the Mitzvah of Lulav applies at night.
Rebbi Eliezer does not need a second phrase to teach that the Mitzvah of Lulav is not performed at night. It seems that according to Rebbi Eliezer, there is no reason to compare Sukah to Lulav, because he maintains that there is no Gezeirah Shavah of "Shiv'as Yamim." However, in the following lines of the Gemara, Rebbi Eliezer says that the reason why the preparations for the Mitzvah of Sukah override Shabbos is because of the Gezeirah Shavah that compares Sukah to Lulav!
If Rebbi Eliezer accepts the Gezeirah Shavah between Lulav and Sukah, then why does he not require another verse to teach that the Mitzvah of Lulav is performed only during the day (and not at night as well, like Sukah)?
(a) TOSFOS (DH Shiv'as) answers that although Rebbi Eliezer agrees that there is a Gezeirah Shavah, he maintains that it cannot be applied when the meaning of the words that are used for the Gezeirah Shavah -- "Shiv'as Yamim" ("seven days") -- preclude the law derived from them! The word "days" cannot teach that just as the Mitzvah of Sukah is performed at night, so, too, the Mitzvah of Lulav is performed at night, because such a teaching would contradict the simple meaning of the words used for the Gezeirah Shavah -- "seven days."
(b) The RE'AH and RITVA answer that Rebbi Eliezer maintains that Sukah and Lulav cannot be compared as far as night is concerned for another reason. With regard to Sukah, the Halachah is that the Mitzvah applies all day and all night. This Halachah cannot be applied to Lulav, because the Torah certainly does not require that the Lulav be held all day and all night. On the other hand, it is unacceptable to suggest that the Gezeirah Shavah teaches that one must pick up the Lulav once at night, because in that case the Halachah with regard to Lulav would not be similar to the Halachah with regard to Sukah, from which the Halachah is derived.
(c) The RASHBA explains that Rebbi Eliezer indeed learns from a verse that the Mitzvah of Lulav does not apply at night. He learns this from the verse "Shiv'as Yamim" (which, incidentally, is the verse used for the Gezeirah Shavah). However, the Rabanan maintain that two verses ("Shiv'as Yamim" and "ba'Yom") are necessary to teach that the Lulav is not taken at night, since there are two different Halachos with regard to the Mitzvah of Lulav that are done only during the day.
The first Halachah is that in the Beis ha'Mikdash, the Mitzvah is to hold the Lulav for seven days. The second Halachah is that outside of the Beis ha'Mikdash, the Lulav is held only the first day. Consequently, one verse is needed to teach that the Lulav is not held at night in the Beis ha'Mikdash, and another verse is needed to teach that it is not held at night outside of the Beis ha'Mikdash. Rebbi Eliezer, though, maintains that the Halachah that the Lulav is not held at night outside of the Beis ha'Mikdash is derived from the Halachah with regard to inside the Beis ha'Mikdash, and thus only one verse is needed to teach that it is not held at night.