Before us is Mishnah 3 in the fifth chapter of Maseches Kesubos. The Mishnah opens with the ruling: "HaYavam eino ma'achil biTerumah" - a yavam does not enable his yevamah to eat Terumah.
A woman whose husband died and who now falls to yibum before her husband's brothers, who are kohanim: as long as the yavam has not performed yibum with her and she is waiting for yibum or chalitzah, the yavam does not enable her to eat Terumah. If she is the daughter of a Yisrael, she does not eat Terumah in her own right but only through her husband; and once the husband died and she fell to yibum, the connection between her and the yavam is no more than a bond of zikah, and that is not enough to permit her to eat Terumah. But once he actually performs yibum with her and she becomes his full wife, she does eat Terumah.
The cases the Mishnah presents:
The Mishnah illustrates just how thoroughly she does not eat under the yavam's authority, and presents a series of scenarios. In all of them we are speaking of a husband with whom she had only kiddushin and not nisu'in:
"Asah shishah chodashim lifnei haba'al v'shishah chodashim lifnei hayavam" - she spent six months after the kiddushin under her husband, and once he died she spent another six months under the authority of the yavam without yibum having been performed.
"Va'afilu kulan lifnei haba'al chaser yom echad lifnei hayavam" - even if all twelve months were before the husband except for one day, and on that day, just before the year was completed, he died and she fell before the yavam.
"O kulan lifnei hayavam chaser yom echad lifnei haba'al" - and all the more so when the entire year passed under the authority of the yavam, and the husband died just one day after the kiddushin and she fell to yibum. As the Gemara says, if in the previous case she does not eat, certainly she does not eat here, since she spent only the briefest time with the husband.
"Ein ochelet biTerumah" - in all of these cases she does not eat Terumah.
The twelve months are not incidental: this is the period at whose end they were supposed to marry. As we learned in the previous Mishnah, once the time for nisu'in arrived and they did not marry, if the delay is on the husband's account, he is obligated in her support and enables her to eat Terumah. Accordingly, had she completed a full year under her husband, she would have begun eating Terumah; but in the scenarios before us one day is missing from the completion of the year, and that is not enough.
From here emerges an additional point the Mishnah wishes to convey: throughout this entire period the woman never ate Terumah at all - not while with the husband, because the twelve months were not completed, and not while with the yavam, because a yavam does not enable her to eat Terumah. Had the twelve months been completed during the husband's lifetime, then even though she would have had to stop after his death, at least at some stage she would have eaten Terumah.
"Zu Mishnah rishonah":
Regarding this ruling and the ruling of the previous Mishnah, the Mishnah says: "Zu Mishnah rishonah" - this is the earlier approach, according to which once the time arrived when they were supposed to marry and the husband is alive, she is supported by him and he enables her to eat Terumah. However, "Beis din shel achareihen amru: ein ha'ishah ochelet biTerumah ad shetikanes lachupah" - the later beis din rejected this approach and ruled that a woman does not eat Terumah until she actually enters the chupah, until she has genuine nisu'in.
The two opinions underlying the ruling:
As we mentioned in the previous Mishnah, there are differing views as to why she is not permitted to eat Terumah on the strength of kiddushin alone, before nisu'in or before the time arrived:
Perhaps a blemish will be found in her that will invalidate the marriage, and it will turn out that she was never fit to eat Terumah at all.
Perhaps she will feed the Terumah to her brothers and sisters.
And we said that once the time arrived when she was supposed to marry, the concerns are resolved: there is no concern that she will feed her relatives, since the husband designates a separate place for her to live because he is supporting her; and according to the other opinion, the husband has already made inquiries and checked with her relatives that she has no blemish.
What did the later beis din add?
The second beis din, which rejected this approach, examined the matter according to both opinions:
According to the opinion that the concern is her feeding her relatives: true, once the time arrived and the husband designates a place for her, that concern is removed, but the later beis din ruled that the problem of a blemish still stands. This opinion now holds that there is a concern of a blemish, and therefore one must wait until she actually enters the chupah.
According to the opinion that the concern is a blemish: according to the earlier Mishnah a blemish is no obstacle once the time for nisu'in arrived, since he checked with her relatives. But according to the later Mishnah that same opinion holds that the initial inquiry with the relatives is not sufficient, since perhaps she has another blemish, a more internal one, that is not detectable. Therefore they said that until she actually enters the chupah she does not eat Terumah.
In summary: in this Mishnah we learned that a yavam does not enable his yevamah to eat Terumah, since there is nothing between them but zikah, and only after he actually performs yibum with her does she eat. We examined the scenarios in the Mishnah, in all of which the woman ate no Terumah at all - not through the husband, since the twelve months were not completed, and not through the yavam. And finally we learned the difference between the earlier Mishnah, which permitted her to eat once the time for nisu'in arrived, and the beis din that came after them, which ruled that a woman does not eat Terumah until she enters the chupah, out of concern for an internal blemish that the inquiry did not reveal.