Gittin, Chapter Seven, Mishnah 6. Our Mishnah deals with a get given on condition that the wife serve the husband's father or nurse his son, and with the question of the get's status when the condition cannot be fulfilled.
The first case - a condition with no time limit set:
A man says to his wife as he hands her the get: "Harei zeh gitech al menas shetishamshi es abba" - "This is your get on condition that you serve my father," or "al menas shetanikee es beni" - "on condition that you nurse my son." The Gemara explains that since he set no time limit at all for fulfilling the condition, it is enough that she perform one of these acts for a single day, and the condition is thereby fulfilled.
The Mishnah, however, wants to clarify one point regarding nursing: up to what age is this still considered nursing in the normal way of an infant? On this the Mishnah asks: "V'chamah hee menikato?" - "And for how long does she nurse him?" That is, not how long she must nurse in order to fulfill the condition, but within what span of time she must do so even for a single day:
Two years - within the first twenty four months.
Rabbi Yehudah says: eighteen months - he disagrees about the time frame, and holds that it must be done within the first eighteen months.
If the son died or the father died:
The Mishnah continues and states that in these circumstances - when the son died or the father died and they are no longer alive for her to serve or to nurse - it is a valid get. The reason: his intent was not to cause her distress, but to gain a benefit, and that benefit is no longer needed once they are not alive.
The commentators disagree in understanding this ruling:
Some understand that the Mishnah is speaking of a case in which she actually fulfilled the condition, and only afterward they died.
Others hold that even if she did not fulfill the condition at all, the get is valid, since there is no longer any need for the condition to be fulfilled and the act would bring the man no benefit.
The second case - a condition with a set time:
He said to her: "Harei zeh gitech al menas shetishamshi es abba shtei shanim" - "This is your get on condition that you serve my father for two years," or "al menas shetanikee es beni shtei shanim" - "on condition that you nurse my son for two years." Here the husband set an explicit time frame. In this scenario, if the son died, or the father died, or the father said that he does not want her to serve him (and this does not stem from any complaint against her, but simply from lack of interest) - it is not a valid get. Since he made the condition depend on two years, the condition must be fulfilled in full, and without its fulfillment the get does not take effect.
The opinion of Rabban Shimon ben Gamliel:
Rabban Shimon ben Gamliel disagrees and holds that in all these cases - the father died, the son died, or the father does not want her to serve him - it is a valid get. Just as was explained in the first case: since they are not here, the husband is not particular about the fulfillment of the condition.
And he stated a general rule: "Kol akavah she'einah heimenah - harei zeh get" - "Any impediment that does not come from her - it is a valid get." Any obstacle that prevents her from being able to fulfill the condition and does not stem from her and her actions does not invalidate the get. After all, she had no part in the father's death, in the son's death, or in the father's not wanting her service, and since she did not cause it, the condition need no longer be fulfilled, and the get is valid even though the condition was not fulfilled.
In summary: In this Mishnah we learned that a condition with no set time is fulfilled by a single day, and regarding nursing, it must be within two years (and according to Rabbi Yehudah, within eighteen months). When the condition became void on its own through the death of the father or the son, the get is valid; but when the husband stipulated two years, the first opinion and Rabban Shimon ben Gamliel disagree, with Rabban Shimon ben Gamliel validating the get on the basis of the rule that any impediment not stemming from her does not invalidate it.