On Kidushin 61a, In reference Tnai Bnei Gad u'Vnei Reuven.
The Mishnah learns from Moshe's Language 3 requirements:
1) Tenai Kaful
2) Tenai Before Maaseh
3) Positive before the negative
The Diyuk of Tenai Kaful is understandable because the Torah wouldn't repeat something unnecessarily. The fact that it repeated the negative shows that you can't learn the negative from the positive.
But what's the Diyuk for the 2nd and 3rd requirements? There's nothing out of the ordinary in putting the Tenai 1st and putting the positive 1st. He had to choose one way. You can't write both of them at the same time. On the contrary the Tenai 1st and the positive 1st is the normal way of speaking.
(My question is based upon Rashi's Perush on the Mishnah 61A.)
1a) Rashi (on the Mishnah, 61a) writes that the fact that Moshe did not say, "Give them if they pass over," suggests that if he would have said this, immediately upon saying "Give them" the matter would have been decided and it would no longer help to say "if they pass over," because once the gift has been granted by saying "Give them" it is no longer possible to apply conditions to this fait accompli.
In other words, if the Ma'aseh would have come first it would already be "done" and a Tenai would not have sufficient power to uproot it.
1b) I found that the Me'iri here explains why one requires the positive before the negative. He writes that if someone puts the negative first, this suggests that he is not being as careful wth his words as you would expect someone to be if he really wanted the Tenai to work. If someone really wants the Tenai to work, he would be careful to make sure that the "Kiyum ha'Ma'aseh" -- mention of the action taking place -- comes first.
2) It seems that if Moshe would have said, "If they do not pass over do not give them," the fact that he has said "Do not give them" cannot be overturned even if afterwards he would say "If they pass over give to them," because once the words "do not give them" have been uttered by Moshe, it is not possible to retract.
Kol Tuv,
Dovid Bloom
Thank you rabbi Bloom. The question is How do we know this ? In other words Rashi says because he put the Tenai 1st and the positive 1st that teaches that thats the only way it works. How does Rashi know that? Maybe if Moshe put the negative 1st it also would have worked and if he put the Maaseh 1st it would have worked?
1) We can distinguish between the Halachah that one has to put the Tenai first and the Halachah that one must put the positive first. Rashi explains clearly why one has to put the Tenai first but it is more difficult to understand from Rashi why one has to put the positive first. Therefore, I will attempt first, bs'd, to explain the easier issue.
2) Rashi writes that if Moshe would have said "give them if they pass over," the Tenai of "if they pass over" would not have been able to annul or modify the "Ma'aseh," the "action," of "give them," the gift that was said prior to the Tenai, and the gift would have been binding regardless of the fulfillment of the condition. Rashi is telling us that a Tenai does not always possess the power to annul an action. This means that once Moshe would have said "give them" first, it is all finished. It is now too late to retract from that gift, even if one would attempt to apply a Tenai that the gift should be binding only if they pass over. This is what Rashi means when he writes "Lo Asi Tenai u'Mevatel Ma'aseh." The Tenai does not possess the power to annul the action.
3) The words of Rashi about the positive coming first are difficult to understand. By way of introduction, I am going to mention what my Rosh Yeshiva, Rav Tzvi Kushelevsky shlit"a, often did in similar situations. He siad that when you learn other Rishonim, you often find that Rashi is saying a Pshat similar to another Rishon. Rashi usually says his Pshat in a much briefer way than other Rishonim, but after one learns the other Rishonim, one sometimes notices that Rashi is hinting at their Pshat in a very concise fashion. Therefore, bs'd I will try to look at other Rishonim here and see if they can help us to understand Rashi.
4) I did find that the Sefer Nachalas Yakov (written by a Talmid of Rebbi Akiva Eiger) gives a Pshat on the words of Rashi here.
a. The Nachalas Yakov, in Parshas Naso, asks your question: One of the two, either the positive or negative, must anyway come first! If the Torah would have put the negative first, then I would have said the opposite, that the negative must always come first in order for the Tenai to be valid. Since there is nothing superfluous in what the Torah says, how do we know that the Torah specifically put the positive first and that otherwise the Tenai will not work?
b. The Nachalas Yakov answers that one must say that if Moshe would have said, "If they do not pass over, do not give to them," this in itself would be superfluous. If they would not go to the effort of passing over the Jordan River and helping their brethren in the war, why should one have thought that they would get anything on the west side of the Jordan? Why would it have even been necessary to say they do not get?
c. In the light of this question, the Nachalas Yakov writes that if Moshe would have said, "If they do not pass over, do not give them," this is not a meaningful statement, because it is unnecessary to say it. Therefore, when he said, "If they do pass over, give to them," this was the first meaningful statement that he said. It follows that there is no double Tenai, because he only said one thing. This means that if Moshe would have said the negative first, there would not have been a double Tenai, and we know that according to Rebbi Meir, if the Tenai is not double it is not considered a Tenai. This is the proof that the positive must come before the negative.
Kol Tuv,
Dovid Bloom