Gemara

(Mishnah): If the Ma'aseh (the event governed by the Tenai) precedes the Tenai, the Tenai is void.

Our Mishnah is like R. Meir.

(Beraisa - Aba Chalifta citing R. Meir): If the Tenai precedes the Ma'aseh, the Tenai is valid. If not, it is void.

Kidushin 60b (Rav Huna citing Rebbi): Saying 'Al Menas (on condition)' is like saying 'from now.'

61a (Mishnah - R. Meir): A Tenai (stipulation) is invalid if it is not (Kaful, i.e. discusses both possibilities) like the Tenai made with the tribes of Gad and Reuven. "If they will cross the Yarden... if they will not cross to fight..."

R. Chanina ben Gamliel says, the verse does not teach this (it is not extra). There was a need to stipulate that if they do not cross, they will inherit in Eretz Yisrael.

Gitin 75a (Beraisa): If one said 'the Get is yours but the paper is mine', she is not divorced. If he said 'Al Menas that you return the paper to me', she is divorced.

Question: What is the difference between the two clauses?

Answer #1 (Abaye): The Beraisa is like R. Meir, who says that a Tenai is invalid unless it is Kaful. (In the Seifa, he did not double the Tenai, so it is Batel, and the Ma'aseh (the act about which he stipulates) takes effect.)

Answer #2 (Rava): In the Seifa, the Ma'aseh preceded the Tenai.

Answer #3 (Rav Ada bar Ahavah): In the Seifa, the Tenai and Ma'aseh involve the same matter.

Answer #4 (Rav Ashi): The Mishnah is like Rebbi, who says that saying 'Al Menas that' is like saying 'from now' (there is no contradiction between the Tenai and the Ma'aseh. She is divorced only if she fulfills the Tenai.)

(Rava): Every Tenai must be like that of Benei Gad, in which the positive (if they will fight) preceded the negative (if they will not fight).

Rishonim

Rif and Rosh (Gitin 37b and 6:10): Kefel is needed only for a Tenai 'if', like Benei Gad. It is not needed for a Tenai 'Al Menas', for 'Al Menas' is like 'from now.'

Rosh: The primary opinion is like I wrote (see 6:9 below, that all Mishpetei Tanayim (conditions for a proper Tenai) are needed even for a Tenai 'Al Menas').

Question (Ran Gitin 36b DH Tanu): Rashi says that Chachamim argue only about Kefel, but all learn from Benei Gad that the Tenai must precede the Ma'aseh, and the Tenai and Ma'aseh cannot be in the same matter. This is unreasonable. R. Meir requires Kefel only due to an extra verse. Perhaps even he does not require these other two conditions, for which there is no extra verse. Perhaps the Tenai of Benei Gad just happened to be like this!

Answer (Ran): Benei Gad mentioned the Ma'aseh first. Surely, Moshe reversed the order because the Tenai must be first. There is no source to say that R. Meir and Chachamim argue about this. The Gemara assumed that a Tenai 'Al Menas that you return the paper to me' is void because 'Al Menas' is unlike 'from now', like Chachamim, so the Tenai contradicts the Ma'aseh. We conclude like Rebbi. 'Al Menas' is like from now, so the Tenai does not contradict the Ma'aseh. The Rif and Rambam do not require Mishpetei Tanayim for a Tenai of 'Al Menas'. Rav Ashi supports this. Why did he say that the Beraisa is (only) like Rebbi? Chachamim agree that the Tenai must precede the Ma'aseh, and that they are about different matters! We must say that a Tenai of 'Al Menas' does not need Mishpetei Tanayim.

Rebuttal (Ran): In Bava Metzia, R. Meir and Chachamim argue about a Tenai before the Ma'aseh. R. Meir holds that the Kefel in Tenai Benei Gad is extra, to require all Tanayim to be just like Tenai Benei Gad. Chachamim say that it is not extra, and we do not learn to all Tanayim. Rav Ashi established the Beraisa like Rebbi, so is even like Chachamim of R. Meir. It can also be R. Meir. It is better to say so, for then she is divorced even if she does not return it, for the Tenai and Ma'aseh are about the same matter. R. Meir requires all Mishpetei Tanayim for a Tenai to be Mevatel a Ma'aseh that did not take effect yet, and all the more so if the Ma'aseh already took effect!

Defense (Magid Mishneh Hilchos Ishus 6:18): A Tenai of 'if' must be stronger, in order to delay the Ma'aseh from taking effect until the Tenai is fulfilled.

Ran (ibid.): R. Yochanan says that 'Al Menas' is like 'from now', yet Chalitzah 'Al Menas that she will give 200 Zuz' is Kosher in any case. The Tenai is Batel, for a Shali'ach could not do the Ma'aseh.

Rambam (Hilchos Gerushin 9:1): If one divorced to take effect after a fixed time, she is divorced when the time comes, like a Tenai. However, here he did not divorce until the time, unlike divorce on Tenai, which takes effect immediately. Therefore, one who divorces on Tenai must make a Tenai Kaful, but one who divorces after a fixed time does not need Kefel or any Mishpetei Tanayim.

Rosh (Gitin 6:8): In other places, the Gemara did not explain that the Mishnah omitted the Mishpetei Tanayim. Everywhere else it relies on our Sugya, which needed to discuss them to resolve the contradiction.

Rosh (9): Regarding a tricked Chalitzah, we find that even when one says 'Al Menas', the Tenai is valid only if a Shali'ach could do the Ma'aseh. I.e., also such Tanayim must be like that of Benei Gad and Benei Reuven. R. Chananel requires all Mishpetei Tanayim even when he said 'Al Menas.' Chachamim of our region agree, and this is the practice.'

Poskim

Shulchan Aruch (EH 38:3): If one said 'Al Menas' or 'from now' he does not need Kefel or the Tenai before the Ma'aseh. Some say that he needs all Mishpetei Tanayim (also the positive before the negative, and it is possible to fulfill).

Beis Yosef (DH veha'Rosh): The Rosh requires the Tenai and Ma'aseh to involve different matters. The Rambam disagrees, for he (Hilchos Gerushin 8:7) says that she is divorced only if she returns the paper.

Dagul me'Revavah and Hagahosav on Beis Yosef (brought in Rosh Pinah Shulchan Aruch): Also the Tur (Siman 143) says that she is divorced only if she returns the paper, yet he holds like the Rosh! Primarily, the Beis Yosef infers that the Rambam disagrees because he did not mention this among Mishpetei Tanayim in Hilchos Ishus 6:1,2. However, the Rambam did not mention in Hilchos Ishus that a Shali'ach must be able to do the Ma'aseh. He relied on what he wrote in Hilchos Gerushin. The Beis Yosef proves from Hilchos Gerushin we cannot say similarly about a Tenai and Ma'aseh involving the same matter.

Beis Shmuel (4): The Rosh and Ran say that the Poskim who do not require Kefel with a Tenai of 'Al Menas' or 'from now' also allow a Tenai and the Ma'aseh to involve the same matter. We cannot infer whether or not the positive must precede the negative. However, the Ran always requires ending with the negative.