(Mishnah): If Reuven and David have two brothers each, one brother of Reuven does Chalitzah with (e.g. Leah) and one brother of David does Chalitzah with Rachel. Reuven's other brother does Yibum with Rachel and David's other brother does Yibum with Leah;

If both of Reuven's brothers did Chalitzah, Shimon's brothers may not both do Yibum. One must do Chalitzah and the other may do Yibum. If both did Yibum, we do not take [their wives] from them.

(Gemara): This is only for Yisrael, but for Kohanim, it is forbidden. (TOSFOS HA'RID - here, each woman received Chalitzah from a different brother. There is no Heker to inform people that each Chalitzah was only Safek. Therefore, a Kohen may not do Yibum, lest people come to permit a Vadai Chalutzah to a Kohen.)

(Mishnah): The Mitzvah is for the oldest brother to do Yibum. If a younger brother did Yibum, it is valid.

If one was suspected of Bi'ah with a slave and she was freed, or about a Nochris and she converted, he may not marry her. If he did, we do not take her from him;

If one was suspected of [Bi'ah with] a married woman, and they (Beis Din) made her husband divorce her, even if he married her; he must divorce her.

(Gemara): The Mitzvah is for the oldest to do Yibum, for it says "v'Hayah ha'Bechor Asher Teled";

Question: Whom does this refer to?

If it refers to the baby born - it should say "'Yakum Al Shem Achi Aviv ha'Mes"!

Answer #1: It means that [Yibum applies only if] the child born [would have been] the Bechor [if he was born to the Mes, i.e. the Mes had no children].

Objection: If so, even if the Mes had children and they died afterwards, his widow would not be Zekukah to Yibum. [If this were true,] the verse should have said 'and there were not children to him'!

Answer #2: Rather, if the verse does not refer to the baby born, use it to teach about the one who does Yibum - the Mitzvah is for [the Bechor, i.e.] the oldest to do Yibum.

(Mishnah): If one was suspected of Bi'ah with a slave and she was freed, or about a Nochris and she converted, he may not marry her.

If he was Mekadesh her, it is as if he married her (he need not divorce her; he may make Nisu'in with her).

If [Beis Din] told him not to marry her, and he married her, we take her from him (he must divorce her).

Question: If he [transgressed and married her, and] divorced her, may he remarry her?

Answer: If you will say so (that he may not remarry her), you cast suspicion on her children [before divorce. People will think that she was forbidden to him, and her children are illegitimate!]

(Mishnah): If one was suspected of [Bi'ah with] a married woman...

(Rav): He was suspected through witnesses [who saw Zenus].

Question (R. Yosi): If Ploni was suspected through witnesses, would a Beraisa teach "if they made him (her husband) divorce her, even if [Ploni] married her, he must divorce her, and from someone else (if she remarried, and was widowed or divorced, and Ploni married her) he can keep her"?!

If he was suspected through witnesses, even from someone else, he must divorce her!

Answer: Rather, the case is, he was suspected not through witnesses. If they made [her husband] divorce her, [and she remarried, and was widowed or divorced, and Ploni married her,] he can keep her.