Kesubos Chapter 7, Mishnah 4. The Mishnah opens with the words "HaMadir es ishto shelo teileich l'veis aviha" - one who forbids his wife by vow from going to her father's house. On the surface this describes a husband who makes a vow against his wife, but the Gemara explains - as we have already seen in the two previous Mishnayos - that the case is one in which the wife herself made the vow, and the husband upholds it rather than annulling it.
What is the vow the Mishnah is discussing:
A husband may not annul just any vow, but only a vow that involves affliction of the soul - something that causes his wife distress - or a vow that touches on matters between the two of them. The Gemara therefore explains that the vow here is that she forbade upon herself any benefit from marital relations with him if she goes to her father's house. Such a vow he has the power to annul, and since he chose not to annul it but to uphold it, he is obligated to divorce her and pay her kesubah.
When her father's house is in the same city:
The Mishnah qualifies: "bizman shehu imah ba'ir" - when her father's house is located in the same city, and she is therefore accustomed to going there frequently. Under these circumstances the measure is set in months:
"Chodesh echad - mekayem" - if she vowed not to go to her father's house for one month, he is not obligated to divorce her, since she can bear this for a month's time.
"Shnayim - yotzi v'yiten kesubah" - if she vowed to stay away from her father's house for more than a month, he must divorce her immediately and pay her kesubah.
An open-ended vow: if she vowed without specifying any set period of time, here too he must give her a get and her kesubah immediately.
When her father's house is in another city:
When her father lives in another city, the criterion is not based on months but on regalim, that is, on the festivals of Israel: "regel echad - mekayem, sheloshah - yotzi v'yiten kesubah". If the vow is that she not go for one festival, he is not obligated to divorce her; and if she vowed not to go for three festivals, he must divorce her and pay her kesubah.
The Gemara explains, however, that the jump from one month to three festivals cannot be understood at face value, and words must be added to the Mishnah. The addition is the one we have already mentioned in several earlier Mishnayos: there is a difference between the wife of a kohen and the wife of a Yisrael, in that the wife of a kohen is given a longer period. The Mishnah is therefore to be read as follows:
The wife of a Yisrael: one festival - he may uphold the vow and is not obligated to give her a get; more than one festival - he divorces her and pays the kesubah.
The wife of a kohen: two festivals - he may uphold the vow and is not obligated to divorce her; three festivals - he divorces her and pays her kesubah.
In summary: in this Mishnah we learned that the case is one of a woman who vowed not to derive benefit from her husband if she goes to her father's house, and a husband who upholds her vow rather than annulling it. When her father's house is in the same city, the matter is measured in months - one month he may uphold, and beyond that (as well as a vow with no set time) he must divorce her and pay the kesubah. When her father's house is in another city, the matter is measured in festivals, and according to the Gemara's explanation we must distinguish between the wife of a Yisrael, for whom the measure is one festival, and the wife of a kohen, for whom the measure is two festivals.