Before us is Mishnah 9 in the eleventh chapter of Maseches Nedarim. The Mishnah opens by quoting a verse from the passage of vows, which states that the vow of a widow or a divorced woman stands and is not subject to annulment. At first glance this seems self evident and hardly worth stating: if she is a widow or a divorcee, she has no husband, and if she has no husband there is no one to annul her vow, so of course the vow remains in force. What, then, does the verse come to teach us? That is the question of the Mishnah.
The words of the Mishnah: "V'neder almanah u'gerushah... yakum aleha" - the vow of a widow or a divorced woman is upheld and remains in force. (The verse itself also says "all that she has forbidden upon herself," and those words the Mishnah does not quote.) And the question still stands: it is obvious that she has no husband, so what is the novelty here?
The novelty: a vow made before the marriage:
The Mishnah explains that the verse comes to teach a particular case: "Hareini nezirah l'achar sheloshim yom" - a woman who has no husband takes a vow of nezirus that will take effect in thirty days. "Af al pi shenis'es b'toch sheloshim yom - eino yachol l'hafer". That is, even if she married within those thirty days, so that at the moment the vow actually took effect she was already married, the husband cannot annul the vow, because the vow was made at a time when she was not yet under his authority - when she was single, a widow or a divorcee. This is the novelty that the verse comes to teach us.
She vowed while under her husband's authority:
The Mishnah continues with the reverse case: "Nadrah v'hi bir'shus haba'al - meifer lah". A woman who took a vow while she was under her husband's authority, even if she afterward left his authority, her husband can annul it. How so? She said "Hareini nezirah l'achar sheloshim" while still married, and her husband annulled the vow for her. "Af al pi shenis'almenah o shenis'garshah b'toch sheloshim - harei zeh mufar". Even if she was widowed or divorced within those thirty days, before the vow took effect, the annulment stands, since everything depends on the moment the vow was made: if at that moment she was married, the annulment takes effect.
A vow from a previous marriage:
Another case: "Nadrah bo bayom, nis'garshah bo bayom, hecheziroh bo bayom - eino yachol l'hafer". She took a vow on a given day, was divorced on that same day, and her husband remarried her on that very same day - even so he cannot annul it. Although it is all within the same day, and ordinarily a husband can annul a vow within that same day, here the vow was made within the framework of the previous marriage. And this is the rule: a husband does not annul vows from a prior status - even though it is his own marriage, he does not annul a vow that she took during a previous marriage.
The rule that emerges from the Mishnah:
"Kol sheyatz'ah lir'shus atzmah sha'ah achas - eino yachol l'hafer". Any woman who left to her own authority and stood under her own authority for even one hour, and even for a single moment in the middle of the day, her husband can no longer annul her vow.
In summary: in this Mishnah we learned that the validity of an annulment is determined by the woman's status at the moment she took the vow, and not by the moment the vow actually took effect: if she vowed while single and married afterward, the husband does not annul it, even though the vow takes effect during their marriage; if she vowed while married and her husband annulled it, and afterward she was widowed or divorced, the annulment stands. We also learned that a husband does not annul vows from a prior status, even within his own marriage, and that once she has been under her own authority for even one hour, he can no longer annul her vows.