1) "LEIMOR" MEANS "LAV EMOR"
QUESTION: The Gemara quotes a Beraisa in which Rebbi Yehudah teaches that when a Parshah in the Torah starts with the word "Leimor" ("Hash-m said to Moshe, saying"), its intention is to say "Lav Emor" -- the Mitzvah which the Torah is about to teach includes a negative commandment, a Lav, as well.
According to Rebbi Yehudah, almost every Mitzvas Aseh in the Torah should also be a Lav, since the word "Leimor" is used to introduce hundreds of passages. Why is every Mitzvas Aseh that is introduced by the word "Leimor" not also a Lav?
ANSWERS:
(a) The SHITAH MEKUBETZES in Temurah (7a, #6) answers that the Gemara means that only when another Lav is written in the same Parshah as the Aseh does the word "Leimor" teach that the Aseh in the Parshah is also a Lav.
(b) TOSFOS here (DH Leimor) and the RA'AVAD (quoted by the Shitah Mekubetzes) answer that according to Rebbi Yehudah, when the word "Leimor" appears at the beginning of a Parshah, a person receives Malkus only when the Mitzvah in that Parshah is a "Lav ha'Ba Michlal Aseh," a negative prohibition that is expressed as a Mitzvas Aseh.
(c) RABEINU CHANANEL explains that according to Rebbi Yehudah, whenever the Torah says "Leimor," it indeed makes every Mitzvas Aseh in that Parshah into a Lav.
The LECHEM MISHNEH (Hilchos Erchin 5:6) explains that the word "Leimor" is only an "Asmachta" for a Lav. Accordingly, Malkus mid'Oraisa will not be administered for the transgression of a Mitzvas Aseh even according to Rebbi Yehudah. This might also be the intention of Rabeinu Chananel.
2) HALACHAH: KNEADING DOUGH FOR MATZAH WITH WARM WATER
OPINIONS: Rav Yehudah states that when one kneads dough for Matzah, he must use only "Mayim she'Lanu," water that rested overnight (that is, it was drawn from its source and left overnight in its container). Water that is left to rest overnight loses its natural warmth and its potency to cause dough to become Chametz. Rava adds, similarly, that a woman should not knead dough for Matzah in the sun or with warm water.
The Gemara asks what the status of the Matzah is in a case in which a woman kneaded the dough with warm water, in a manner which was proscribed? Rav Ashi says that the Matzah made from such dough is forbidden to be used on Pesach, and Mar Zutra permits it. What is the Halachah?
(a) The RIF rules stringently and says that even if she merely kneaded the dough in the sun it is forbidden, and it is certainly forbidden if she kneaded it with warm water. The Rif's words imply that such Matzah may not be used throughout all of Pesach, since it may contain Chametz. The ROSH explains that the Rif's ruling also applies to a case in which the dough was kneaded with water that was not left overnight. Even in such a case, the Matzah is forbidden.
(b) The ROSH writes that according to RASHI, when the Gemara mentions Matzah made in an improper manner, it refers only to Matzah made from dough that was kneaded in the sun or kneaded with warm water, but not to Matzah kneaded with water that did not rest overnight. In the latter case, the Matzah is certainly permitted b'Di'eved, because the requirement to use Mayim she'Lanu is only l'Chatchilah. (Presumably, water that was not left to rest overnight is not warm enough to cause any serious doubt whether the Matzah became Chametz.) Therefore, the Rosh rules that Matzah made from dough kneaded in the sun or with warm water is prohibited (like the Rif), but dough kneaded with water that did not rest overnight is permitted b'Di'eved (not like the Rif).
(c) The RAMBAM (Hilchos Chametz u'Matzah 5:1-2) differentiates between the cases in the following way. Matzah made from dough that was kneaded with warm water or with water that did not rest is prohibited. In contrast, Matzah made from dough kneaded in the sun is permitted b'Di'eved. The Rambam distinguishes between whether the problem is in the dough itself (such as the water used in the dough), in which case the Matzah is prohibited, or whether the problem is external (such as the sun in which the dough was kneaded), in which case the Matzah is permitted b'Di'eved.
HALACHAH: The SHULCHAN ARUCH (OC 455:3 and 459:6) rules in accordance with the opinion of the RAMBAM and writes that Matzah made from dough kneaded with warm water or with water that did not rest overnight is prohibited even b'Di'eved, and Matzah made from dough kneaded in the sun is permitted b'Di'eved.
The Rishonim give different reasons for why Matzah made from dough kneaded with warm water is prohibited.
1. The ROSH infers from RASHI here (DH Heichi Itmar) that the Rabanan instituted a penalty for one who does not follow their enactment that cool water be used. Even when one transgresses the enactment of the Rabanan inadvertently, the penalty still applies in this case, because it is considered to be "Karov l'Mezid," close to intentional transgression.
2. The HAGAHOS ASHIRI cites the YERE'IM who agrees that the prohibition is a penalty, but he maintains that the penalty applies only to one who intentionally transgresses the enactment of the Rabanan.
3. The Rosh cites the BA'AL HA'ITUR who says that the prohibition to use Matzah made with warm water and the like is not a penalty, but it is an intrinsic disqualification in the Matzah. In order for Matzah to be valid for the Mitzvah, it must have proper "Shemirah" -- that is, it must be guarded with extra stringency to prevent it from becoming Chametz during its production. If warm water was used to knead the dough, or if it was kneaded in the sun, then the Matzah lacks the required degree of Shemirah. In such a case, the Matzah that is produced is not considered "Shemurah Matzah" and may not be used for the Mitzvah to eat Matzah on the first night of Pesach. However, it is not Chametz and it may be used during the rest of Pesach.
The Halachah follows the view of Rashi who says that Matzah made from dough kneaded with warm water is prohibited throughout all of Pesach, even if it was kneaded inadvertently with warm water.