Nedarim, chapter seven, mishnah nine. This mishnah deals with a vow in which a husband forbids his wife to derive benefit from him, where the taking effect of the vow depends on a condition - if she goes to her father's house - and on a set period of time.
The first case - "she'at neheneis li ad haPesach, im teilchi l'veis avich ad heChag":
We are standing before Pesach, and the husband tells his wife that she may not derive benefit from him until the time of Pesach, if she goes to her father's house at any time during the whole period up until Sukkos - that is, if she visits her father's house between now and the Festival, even though that time is later than Pesach.
She went before Pesach: By doing so she activated the force of the vow, and therefore she is forbidden to derive benefit from him until Pesach, since that is the scope of the vow - a prohibition of benefit until Pesach.
She went after Pesach: This trip too activates the vow, and she transgresses "bal yacheil devaro". If she derived benefit from her husband earlier, before Pesach, the vow had not yet been activated at that time; it was activated only by her going after Pesach and before Sukkos. It turns out that that very trip to her father's house after Pesach is the transgression of "bal yacheil devaro", meaning the profaning of the spoken word and the violation of the vow. It is worth noting that this is not her own statement but her husband's, and nevertheless she is in the transgression of violating a vow: she was forbidden to derive benefit from her husband, but in practice she transgressed only from the moment she activated the vow by going after Pesach.
The decree of the Sages:
The Gemara explains that on a rabbinic level she is forbidden to derive benefit. In a case where she has not yet derived benefit from her husband, she is forbidden to derive benefit from him until Pesach, a decree because of the concern that she may in the end go after Pesach. Therefore she cannot say, "I will derive benefit now and I will not go after Pesach," since there is a concern that she will go after Pesach and thereby activate the vow. The second case in our mishnah deals specifically with a woman who already derived benefit and afterward activated the vow - in which case she has transgressed the vow.
The second case - "she'at neheneis li ad heChag, im teilchi l'veis avich ad haPesach":
Here the husband says that she is forbidden to derive benefit from him until Sukkos, if she goes to her father's house before Pesach.
She went before Pesach: She activated the vow by going, and therefore she is forbidden to derive benefit from him until Sukkos.
Going after Pesach: This is permitted to her, since the condition itself was stated only regarding going to her father's house before Pesach. After Pesach she may go to her father's house, and doing so activates nothing.
In summary: In this mishnah we learned two formulations of a conditional vow. When the time of the prohibition of benefit is shorter than the time of the condition ("ad haPesach... im teilchi ad heChag") - going before Pesach activates the vow and forbids her to derive benefit until Pesach, and going after Pesach places her in the transgression of "bal yacheil devaro"; and on a rabbinic level benefit was forbidden to her from the outset, a decree lest she go after Pesach. And when the time of the prohibition of benefit is longer than the time of the condition ("ad heChag... im teilchi ad haPesach") - going before Pesach forbids her to derive benefit until Sukkos, whereas after Pesach she is permitted to go to her father's house.