Yevamos, Chapter Seven, Mishnah 4. The Mishnah opens with a list of people and relationships whose power is at once double and limited: they disqualify but do not enable eating. That is, under certain circumstances they have the power to disqualify a woman from eating Terumah, while under other circumstances that very same connection is not enough to render her fit and enable her to eat Terumah. We will go through each case and clarify its meaning.
"They disqualify but do not enable eating":
"HaUbar" - the fetus. A daughter of a kohen who married an ordinary Israelite, and her husband died leaving her pregnant by him: the fetus disqualifies her from returning to her father's house and eating Terumah. Conversely, the daughter of an Israelite who married a kohen, and he died leaving her pregnant by him: a child born to her from that kohen enables her to eat Terumah, but as long as it is still a fetus in its mother's womb it does not enable her to eat until it is born.
"VehaYavam" - and the yavam. A daughter of a kohen who married an Israelite and he died, whose way it is to return to her father's house, and she fell before an Israelite yavam: the zikah of the yavam, the very bond between them, disqualifies her from eating Terumah in her father's house. Conversely, the daughter of an Israelite who married a kohen and he died, and she fell before his brother who is a kohen: even though a bond exists between the two, that potential connection of yibum is not enough to enable her to eat Terumah. Had he actually performed yibum and become her husband, she would eat; but as long as it is only a potential yibum, the yavam does not enable her to eat Terumah.
"VehaErusin" - and betrothal. A daughter of a kohen who was betrothed to an Israelite: from the moment of betrothal to one who is not a kohen she is disqualified from eating Terumah. Conversely, the daughter of an Israelite who was betrothed to a kohen: although by Torah law she is permitted to eat Terumah, the Sages said that a betrothed woman does not eat until she enters the chuppah and is actually married, lest she feed Terumah to her brothers and sisters, since she is still living in her father's house and not in her husband's house.
"VehaCheresh" - and the deaf-mute. A daughter of a kohen who married a deaf-mute Israelite: even though his betrothal is not a Torah-level betrothal, the Sages instituted that he has a rabbinic betrothal, and this betrothal disqualifies her from Terumah, since her husband is an Israelite. Conversely, the daughter of an Israelite who married a deaf-mute kohen: since his betrothal is not a Torah-level betrothal, he does not enable her to eat Terumah.
"UVen tesha shanim veyom echad" - and a boy nine years and one day old:
Once a boy reaches the age of nine years and one day, his cohabitation is considered a valid act of cohabitation; below that age his cohabitation counts for nothing. Therefore, if he is a chalal or a mamzer, or anyone whose status disqualifies the daughter of a kohen and makes her a zonah or a chalalah - his cohabitation is a full act of cohabitation and disqualifies her from Terumah. Conversely, the daughter of an Israelite who cohabited with a kohen who is nine years and one day old: even though his cohabitation is a valid act, his act of acquisition and his sanctity count for nothing, and therefore he does not enable her to eat Terumah.
The cases of doubt:
"Safek shehu ben tesha shanim veyom echad safek she'eino" - it is uncertain whether he is nine years and one day old or not, and the doubt is therefore whether his cohabitation is a valid act. In a case where he is a mamzer or a chalal, a status that disqualifies her from Terumah, we rule stringently and say that she has been disqualified from eating Terumah.
"Safek heivi shtei se'aros safek shelo heivi" - a boy who is thirteen years and one day old, and it is not known whether he has grown two hairs and entered adulthood. From here we see that the doubt operates in both directions: if he is an Israelite who betrothed the daughter of a kohen, perhaps he is an adult and his betrothal is valid, and she is therefore disqualified from Terumah; and if he is a kohen who betrothed the daughter of an Israelite, perhaps his betrothal is not valid, and she therefore does not eat Terumah.
A further case of doubt - the collapse of the house:
The Mishnah concludes with a case that has nothing to do with Terumah, but is another instance of doubt: "Naflah bayis alav ve'al bas achiv" - a man who was married to his niece, and the house collapsed on them and both died, "ve'ein yadua eizeh meis rishon" - and it is not known which of them died first. That man had another wife, the co-wife of the niece, and the question concerns her: if the niece died first, then at the moment of the husband's death there is no longer any forbidden relative here, and the co-wife is not the co-wife of a forbidden relative. But if the husband died first, then at the moment of his death his niece was alive, and she is a forbidden relative to his brothers before whom she would fall for yibum, so the co-wife is the co-wife of a forbidden relative. And since the matter remains in doubt, her co-wife performs chalitzah and does not undergo yibum.
In summary: in this Mishnah we learned the principle of "they disqualify but do not enable eating": the fetus, the yavam, betrothal and the deaf-mute - each one of them disqualifies the daughter of a kohen from Terumah, yet has no power to enable the daughter of an Israelite to eat Terumah. We also examined the case of a boy nine years and one day old, whose cohabitation is a valid act that disqualifies but whose acquisition does not acquire in order to enable eating; the cases of doubt - a doubt whether he is nine years old and a doubt whether he grew two hairs, in which we rule stringently in both directions; and finally the case of the house collapsing on a man and on his niece, where because of the doubt her co-wife performs chalitzah and does not undergo yibum.