Gemara

61a (Mishnah - R. Meir): A Tanai (stipulation) is invalid if it is not (Kaful, i.e. discusses both possibilities) like the Tanai made with the tribes of Gad and Reuven is invalid. "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.

62a - Question: It says "if a man did not lie with you... Hinaki (you will be safe)." R. Chanina can say that we infer that if she is guilty, she will die. According to R. Meir, it should also say 'if you lied with a man, Chinaki (you will be choked)'!

Answer: "Hinaki" is written without a Yud. It also alludes to 'Chinaki'.

Gitin 46b (Mishnah - R. Yehudah): If Reuven divorced his wife because she is an Ailonis (she will not develop like a normal female), he may never remarry her;

Chachamim say, he may remarry her.

(Rava): Chachamim hold like R. Meir, who requires a Tanai Kaful. He did not double the Tanai, so he cannot say that the divorce was mistaken, and is invalid.

75a (Beraisa): If one said 'this is your Get Menas that you return the paper to me', she is divorced.

Answer #1 (Abaye): This is like R. Meir, who requires Tanai Kaful.

Answer #2 (Rava): The Tanai is Batel, for it is after the Ma'aseh.

Answer #3 (Rav Ada bar Ahavah): The Tanai and Ma'aseh involve the same matter.

Answer #4 (Rav Ashi): This is like Rebbi, who says that saying 'Al Menas that' is like saying 'from now.' (The Tanai does not contradict the Ma'aseh, so it is valid.)

Bava Metzi'a 94a (Mishnah): If the Ma'aseh precedes the Tanai, the Tanai is void.

Our Mishnah is like R. Meir (who says in a Beraisa that a Tanai is valid only if it precedes the Ma'aseh).

Bava Basra 137b (Rava): If one said 'take this Esrog Al Menas that you return it to me', and the recipient took it, he was Yotzei only if he returned it afterwards.

Rishonim

Rambam (Hilchos Zechiyah 3:7): Every Tanai for gifts or sales requires Kefel, the positive before the negative, the Tanai before the Ma'aseh, and a Tanai that can be fulfilled. If any is missing, the Tanai is Batel. It is as if there is no Tanai.

Rambam (8): My Rebbeyim require Kefel and putting the positive before the negative only for divorce and Kidushin. There is no proof for this.

Rambam (Hilchos Ishus 6:14): We learn Kefel and the other four Mishpetei Tanayim from the Tanai with Benei Gad and Benei Reuven.

Magid Mishneh: The Ramban says that we always require the Tanai before the Ma'aseh, like the Stam Mishnah (Bava Metzi'a 94a) amidst clear-cut Halachah. It is like R. Meir, but we hold like him about this. Presumably, if we learn from Benei Gad, we should learn everything, but the Gemara refutes this. All Poskim require a Tanai that can be fulfilled. The Rambam requires the Tanai to precede the Ma'aseh, like R. Meir, and rules like him also for Kefel and the positive before the negative. We do not learn halfway. The early Ge'onim agree.

Ran (Kidushin 25b DH Rebbi): Teshuvas ha'Rif (31) says that surely the Ge'onim (who do not require Mishpetei Tanayim for monetary matters) hold that R. Meir requires Kefel for monetary Tanayim, but the Halachah does not follow him. Surely, the Rif did not mean that we do not require any Mishpetei Tanayim. We require that the Tanai precede the Ma'aseh (like the Magid Mishneh proved), and that a Shali'ach could do the Ma'aseh.

Rosh (Bava Basra 8:48): Rava validates the Tanai about an Esrog. Rava argues with Rav Ada, who requires a Tanai and Ma'aseh about distinct matters.

Ha'Makneh (EH 38:4): Perhaps Rava gave one reason why the Tanai is Batel, and agrees with Rav Ada! Or, Al Menas is like from now, and there is no contradiction between the Tanai and the Ma'aseh!

Rosh (ibid.): Sometimes, the intent is clear without even a Giluy Da'as, e.g. a woman about to marry gives a gift to her son so that her husband will not get her property. Sometimes Giluy Da'as is needed and suffices, e.g. one who sells with intent to move to Eretz Yisrael. Everything else needs a proper Tanai.

Rebuttal (Ritva Kidushin 50a DH v'Agav and 61a DH Kol): R. Meir always requires a proper Tanai. If one divorces due to a vow, ill repute or Ailonis, his intent is clear, yet R. Meir requires Kefel. In many cases we do not require a proper Tanai. This shows that the Halachah does not follow R. Meir. Of all the Mishpetei Tanayim, Chachamim require only that a Shali'ach could do the Ma'aseh, like we say about Chalitzah. It seems that R. Chanina holds like Chachamim. Even if he argues with R. Meir only about Kefel and the positive before the negative, the Halachah follows Chachamim.

Chidushei ha'Ran (Gitin Sof 45b): When one divorces his wife due to ill repute, why does R. Meir hold that it suffices to double the Tanai? He learns from Benei Gad that a full Tanai is required! Rather, this is for Tana'im to be fulfilled in the future. If one divorces due to what already passed, a Giluy Da'as suffices. In any case, R. Meir does not infer the positive from the negative, so he requires Kefel.

Question (Tosfos Kidushin 62a DH Bishlomo): In the curse of Sotah, the negative ("if a man did not lie with you...") precedes the positive!

Answer #1 (Tosfos): What we want (that she is innocent) is the positive.

Answer #2 (Avnei Milu'im 38:5 DH v'Yisyashev): The Ran holds that since the Tanai pertains to the past, even R. Meir requires only Kefel.

Poskim

Shulchan Aruch (EH 38:4): Some say that we require the four Mishpetei Tanayim (Kefel, Tanai before the Ma'aseh, the positive before the negative, and it is possible to fulfill) only when this is a stringency, but not if it is a leniency.

Beis Shmuel (6): The Tur, Shulchan Aruch and Shiltei ha'Giborim say that this opinion does not require any of the four. What is their source? The Rambam says only that Kefel and the positive before the negative are not needed. The Magid Mishneh and Ran require the Tanai before the Ma'aseh, and a Ma'aseh possible through a Shali'ach. Even if we do not learn from Benei Gad, we learn what is logical. It seems that the Rosh agrees.

Chelkas Mechokek (5): The Tur and Tosfos say 'the Tanai and Ma'aseh involve the same matter' when they contradict each other, e.g. 'this coin with which I am Mekadesh you will be mine.' Kidushin 'Al Menas that you return the coin' is not contradictory, for Al Menas is from now. Kidushin money that must be returned never works, even with Al Menas, for it resembles Chalipin. If one said 'this is your Get, and the paper is mine', this is not a Get at all. If one gives a Get or Shtar Kidushin to be valid 'if you return it to me', why are we stringent? It is no better than without a Tanai at all, which is totally void!

Ha'Makneh: The Poskim omit that the Tanai and Ma'aseh must involve different matters. Al Menas is from now, so there is no contradiction. If he said 'if...', we can explain that the Ma'aseh takes effect after the Tanai is fulfilled, so in any case it is invalid! This is the Chelkas Mechokek's question.

Beis Shmuel (7): If an Esrog is given 'Al Menas that you return it', there is no contradiction, yet the Rosh calls this a Tanai and Ma'aseh involving the same matter! Also in Gitin (7:9), regarding Al Menas, he requires that the Tanai and Ma'aseh involve different matters. We can explain the Tur similarly. However, in Siman 143, the Tur validates a Tanai 'Al Menas that you return the Get'! Also, the Tur and Rosh validate a Tanai 'Al Menas that you return the Esrog.' We disqualify Kidushin 'Al Menas to return the money' because it is like Chalipin. If the Tanai is Batel, she need not return anything! The Chelkas Mechokek explains the Tur like Tosfos, and says that here, the stringency (to say that we do not require Mishpetei Tanayim) affects only Kidushei Shtar. Really, if he stipulated 'if you return it...' if we required Mishpetei Tanayim, she would not need to return the money, and it is would not be like Chalipin!

Rema: Some say further that even with all four of these, we also require that the Tanai and Ma'aseh involve different matters. One should be stringent to be concerned for this opinion.