OPINIONS: The Mishnah states that both Rebbi Eliezer and the Chachamim agree that a man cannot uphold his wife's Nedarim before she makes them. They disagree only with regard to whether he may annul her Nedarim she makes them. Rebbi Eliezer maintains that he may annul them because of a Kal v'Chomer: if he may annul and remove a Neder which has already taken effect, certainly he may annul a Neder before it takes effect. The Chachamim argue that the husband cannot annul his wife's Nedarim before she makes them because the verse (Bamidbar 30:14) compares Hafarah to Hakamah and teaches that only when it is possible to do Hakamah (when the Neder exists already) is it possible to do Hafarah.
What is the source for the law (with which everyone agrees) that a husband may not uphold his wife's Nedarim before she makes them?
(a) The RAN (DH Harei Hen) explains that there simply is no logical basis that Hakamah should work before the Neder is even made. On the contrary, since Hakamah works to strengthen an existing Neder, it certainly should not work to strengthen a Neder which does not exist. Since there is no basis or source that Hakamah done before a Neder is made is valid, it must be assumed that such a Hakamah is not valid.
However, the ROSH and TOSFOS (as well as REBBI AKIVA EIGER on the Mishnayos) question the Ran's explanation. If the reason why Hakamah is not valid before the Nedarim are made is that there simply is no logical source that such a Hakamah should work, why do the Chachamim derive from the Hekesh between Hakamah and Hafarah that Hafarah also does not work before the Nedarim are made (and they use the Hekesh to counter the Kal v'Chomer of Rebbi Eliezer)? Since there are logical grounds (the Kal v'Chomer) to say that Hafarah may be done before the Neder is made, the Hekesh should compare Hakamah to Hafarah and teach that Hakamah, too, may be made before the Neder. In this way, the Hekesh will not contradict the Kal v'Chomer.
The TIFERES YERUSHALAYIM (on the Mishnayos) answers that the reason why Hakamah cannot take effect before the Neder is made is that the Hakamah would be considered a Hakamah of a "Davar she'Lo Ba l'Olam." Just as a Neder cannot be made to prohibit someone else from a "Davar she'Lo Ba l'Olam," so, too, one cannot make a Hakamah on a "Davar she'Lo Ba l'Olam."
However, this does not seem to be the intent of the Ran. Furthermore, it is possible to make a Neder to prohibit a "Davar she'Lo Ba l'Olam" to oneself, and thus Hakamah should be comparable to that type of Neder (because the Torah gives the husband the right to be Mekayem the Neder of someone else, his wife).
The CHASAM SOFER and PARASHAS NEDARIM answer that the comparison of Hafarah with Hakamah is not an actual Hekesh. Rather, by placing Hakamah ("Ishah Yekeimenu") before Hafarah ("Ishah Yefeirenu") the verse teaches that the ability to be Mekayem the Neder is a pre-requisite for the ability to be Mefer the Neder. The verse is teaching a specific Halachah about Hafarah, and not a comparison between Hakamah and Hafarah. The Parashas Nedarim cites examples for this type of Limud from TOSFOS (Shevuos 20b, DH Kol, and Pesachim 43b, DH Salka). Although the Ran in the end of the Mishnah refers to the Limud as a "Hekesh," from what he writes earlier (end of 73a) it is clear that he does not consider it a genuine Hekesh. Earlier, when the Gemara says that the word "Osah" (Bamidbar 30:9) teaches that one cannot annul two Nedarim at one time, the Ran explains that one also cannot uphold two Nedarim at one time because the verse says "Lah" (30:15) with regard to the Hakamah of a Neder. The Ran there does not derive this law from a Hekesh of Hakamah to Hafarah. It is also clear from the words of the Ran later (86b, DH l'Meimra) that this Hekdesh is not a genuine Hekesh.
(b) The ROSH and TOSFOS suggest a different source for why a husband cannot be Mekayem his wife's Nedarim before she makes them. The source is a logical argument: since the husband does not know what Nedarim his wife might make, he cannot decide wholeheartedly that he wants to be Mekayem them. She might make such a drastic Neder that he will not want to be Mekayem it. His Hakamah is considered a Hakamah in error (Hakamah b'Ta'us).
The RASHASH asks that according to this logic, a man should not be able to be Mekayem any Nedarim which his wife made in the past if he does not yet know what they prohibit (because when he learns what they prohibit, he might not want to uphold them). However, the Gemara (72b) seems to accept the possibility that a man can be Mekayem his wife's Nedarim which she made in the past, before he hears them.
(c) The RASHASH writes that there is an explicit Derashah in the Sifri which teaches that a husband cannot be Mekayem the Nedarim which his wife did not yet make. This Derashah is derived from the verse, "le'Esor Isar Al Nafsho" (Bamidbar 30:3).