QUESTION: The RAN (79b, DH Ileima) writes that one who makes a Neder with a Tenai of "if" ("Im") intends for his Neder to take effect only after the condition is fulfilled. Hence, if one makes a Neder that, "Fruit should be forbidden to me if I wash," there is no concern that she will eat fruit and then wash and thereby transgress the Neder retroactively, because fruit becomes prohibited to her only after she washes.
The BEIS MEIR (EH 145) challenges this principle from the Gemara in Shevuos (28b) which states clearly that one who swears that he will not eat a certain loaf "if" he eats another loaf, and then he eats the first loaf before he eats the contingency loaf, transgresses his Shevu'ah retroactively. The Gemara there seems to disprove the assertion of the Ran that the Neder takes effect only after the condition is fulfilled.
ANSWER: The Ran himself in Shevuos poses this question on his principle, and he differentiates between the two cases as follows. In the case in Nedarim, the contingency is always valid; whenever she will wash, the Neder will take effect. Therefore, it is assumed that her intent is that the Neder should take effect only after the condition is fulfilled. In contrast, in the case in Shevuos, the Neder merely entails one loaf and the possibility exists that he will eat that loaf first and thereby render the entire contingency (of eating the second loaf) redundant. In such a case, it is assumed that one's intent is that the Neder should take effect retroactively.
QUESTION: The Gemara explains that the case of the Mishnah, in which a woman makes a Neder by saying, "Im Erchatz" ("If I wash"), refers to a case in which she makes a Neder to prohibit herself from washing forever if she washes once. Although she is permitted to wash the first time, doing so will make her Neder take effect and she will then be prohibited to wash forever. Therefore, the husband may annul her Neder immediately, even before she washes the first time and before the Neder takes effect.
Later (89b), Rebbi Nasan and the Chachamim disagree about whether the husband may annul his wife's Neder before it actually takes effect. The ROSH writes that the Gemara here explains the case of the Mishnah only according to the Chachamim. According to Rebbi Nasan, she will have to actually wash the first time in order for the Neder to take effect and for her husband to be able to annul it.
The Rishonim ask in the name of RABEINU YONAH that if the Gemara is explaining the Mishnah's case according to the Chachamim, as the Rosh says, the Gemara's question at the beginning of the Sugya (79b) is difficult to understand. The Gemara asserts at the outset that the case of the Mishnah cannot be one in which she made a Neder to prohibit all fruits upon her "if I wash," because if that is the case, "let her not wash and not become prohibited [from the fruits]," and the husband should not be able to annul the Neder.
What is the Gemara's question? Since the Chachamim maintain that the Neder does not need to take effect in practice in order for the husband to annul it, the husband should be able to annul the Neder in the case the Gemara suggests (where she prohibits all fruits upon her if she washes)!
(a) The RAN writes that even the Chachamim do not allow the husband to annul the Neder before it takes effect, except in a case in which the Tenai itself entails Inuy Nefesh. The Gemara at the outset assumes that abstaining from washing does not entail Inuy Nefesh and therefore it asks that even according to the Chachamim "let her not wash and not become prohibited," since there is no Inuy Nefesh involved in not washing.
However, the Ran questions this approach from the Halachah that when a woman accepts upon herself Nezirus only to take effect after she is divorced, her husband may annul the Neder even though there is no Inuy Nefesh yet.
The Ran answers that since the fulfillment of the Tenai is not in her control (since her husband may divorce her any time he wants), it is considered a Neder of Inuy Nefesh. In contrast, in the case of a Neder which is conditional and the condition does not entail Inuy Nefesh, and it is within her power to avoid the Inuy Nefesh, her husband cannot annul the Neder until it actually takes effect, even according to the Chachamim.
The RASHBA challenges the Ran's answer from the ruling of Rebbi Yochanan ben Nuri. When a woman makes a Neder to prohibit her husband from benefiting from her earnings, even though such a Neder does not take effect because she is obligated to provide her husband with her earnings, he still must annul her Neder lest he divorce her and the Neder prevent him from remarrying her. From Rebbi Yochanan ben Nuri's ruling it is clear that although neither the Neder nor the condition causes Inuy Nefesh now or comes between them now, the husband still may annul the Neder!
The Rashba answers this question and says that the case of Rebbi Yochanan ben Nuri is not comparable to an ordinary case of a Neder with a Tenai. In Rebbi Yochanan ben Nuri's case, the Neder takes effect immediately; it is merely her obligation (Shibud) to her husband which inhibits the Neder. Therefore, the husband may annul the Neder before the Isur is actually relevant in practice.
The Rashba continues to question the approach of the Ran. Although the Tenai of not washing might not entail Inuy Nefesh, it definitely is a matter between the two of them, Devarim she'Beino l'Beinah. Therefore, although the Inuy Nefesh element has not yet taken effect, the husband should be able to annul the Neder because the fulfillment of the Tenai causes a rift between them (since she is not bathing)!
(b) The RASHBA instead concludes that the original question of the Gemara, "Let her not wash and not be prohibited," is based on the assumption that the Tana of the Mishnah follows the view of Rebbi Nasan (see the Rashba for further discussion of this issue).