(Mishnah): Reuven, Shimon and Levi (brothers) were married to Rachel and Leah (sisters) and Esther (an unrelated woman), respectively. Reuven died. Levi did Yibum, then he died. Rachel is exempt (from Yibum and Chalitzah) because she is Achos Ishto to Shimon (his wife's sister). Esther is exempt because she is Tzaras Ervah.

Had Levi not done Yibum, but given a Ma'amar and then died, Esther would do Chalitzah, but not Yibum.

(Gemara) Inference: The Seifa depends on the Ma'amar. If not for the Ma'amar, she could do Yibum!

(Rav Nachman): This teaches that Ein Zikah, even when there is one Yavam. (If Yesh Zikah, she would be Tzaras Ervah through Zikah!)

(Mishnah): Reuven and Shimon were married to Rachel and Leah (sisters) and Levi was married to Esther. Levi died. Reuven did Yibum, then he died. Rachel is exempt because she is Shimon's Achos Ishto; Esther is exempt because she is Rachel's Tzarah.

Had Reuven not done Yibum but given a Ma'amar before he died, Esther would do Chalitzah, but not Yibum.

(Gemara) Question: We already know this from the Reisha (the previous Mishnah)!

In the Reisha, Achos Ishto was a Tzarah to Esther (i.e. she married Levi after Esther), and Esther was forbidden. Here, Esther is a Tzarah to Achos Ishto, all the more so Esther is forbidden!

Answer: The Tana first taught the Seifa, thinking that in the Reisha, Esther would be permitted. Later, he realized that even she is forbidden even in the Reisha.

Since the Reisha is the greater Chidush, it is dear to the Tana, so he taught it first. The Seifa (even though now it is not needed) was retained in the Mishnah.

(Mishnah): Reuven and Shimon were married to Rachel and Leah (sisters) and Levi was married to Esther. Reuven died. Levi did Yibum. Leah died, then Levi died. Rachel is forever forbidden to the Yavam, since she was once forbidden.

(Gemara - Rav Yehudah): If we cannot apply "Yevamah Yavo Aleha" when a Yevamah falls to Yibum, she is like Eshes Ach when the brother died with children; she is forbidden.

Question: The Mishnah teaches this!

(Mishnah): She is forever forbidden to the Yavam since she was once forbidden.

Answer: One might have thought that this is only when the Isur remained throughout the first fall to Yibum; but here the Isur went away during the first fall to Yibum, so she is permitted!

Rav Yehudah taught that this is not so.

Question: Also this we learn from a Mishnah!

(Mishnah): Two brothers were married to sisters. One brother died, and then the other brother's wife died. The Yevamah is forever forbidden to the Yavam, since she was once forbidden.

Answer: There, she could not do Yibum with any brother (when she fell). Here, when Rachel fell she could have done Yibum with Levi (but was forbidden to Shimon). Therefore, one might have thought that when the Isur to Shimon (Achos Ishto) goes away, she is permitted to him;

Rav Yehudah teaches that this is not so.

(Mishnah): Reuven and Shimon were married to Rachel and Leah (sisters) and Levi was married to Esther. Reuven divorced Rachel, Levi died, Reuven did Yibum, then he died;

In this case they said that 'If an Ervah died or was divorced, the Tzaros are permitted'.

(Gemara) Inference: Esther is permitted because the divorce preceded Levi's death. Had he died before, Esther would be forbidden.

(Rav Ashi): This shows that Yesh Zikah, even when there are two brothers. (When Levi dies, due to Zikah we immediately consider Esther to be Shimon's wife. When Shimon dies she is considered Tzaras Ervah to Reuven, for through Zikah she was the Tzarah of Achos Ishto (Leah, before the divorce).)

Question: Above, Rav Nachman deduced that Ein Zikah!

Answer: Rav Ashi explains that in that Mishnah, she cannot do Yibum even if no Ma'amar was given;

It mentions Ma'amar to teach unlike Beis Shamai, who say that Ma'amar fully acquires.

Question: Rav Ashi's inference from our Mishnah disproves Rav Nachman!

Answer #1: Even if Levi died before the divorce, Esther is permitted;

'This is the case' excludes when the Yibum precedes the divorce (but not when death precedes the divorce).

Question: This is fine if Rav Nachman holds like R. Yirmeyah, who said 'These Mishnayos (our Mishnah and 2a) must have different authors';

The Tana of 2a holds that death causes the fall to Yibum (a Yevamah is considered Tzaras Ervah only if she was Tzaras Ervah when her husband died).

Our Tana holds that the initial marriage causes the fall (if she was Tzaras Ervah at any time during her marriage, she is considered Tzaras Ervah when she falls). If the Yibum preceded the divorce, since Leah was once Tzaras Ervah, even divorce of the sister will not allow Yibum later.

Rava said that the same Tana taught both, and the Mishnah on 2a is the bigger Chidush (death causes the fall to Yibum, so even if the divorce was after Yibum the Tzarah is permitted).

Summation of question: If Rav Nachman holds like Rava, what does 'This is the case' exclude?

Answer: Rav Nachman must hold like R. Yirmeyah.

Question: Granted, if Rava holds like Rav Ashi, 'This is the case' excludes if he never divorced the sister (even if he never did Yibum, Leah is forbidden, for she is Tzaras Ervah through Zikah);

However, Rava holds like Rav Nachman, what case is excluded?

Answer: Rava must hold like Rav Ashi.

(Mishnah): If an Ervah was Safek Mekudeshes (to a brother) or Safek divorced, the Tzaros do Chalitzah, not Yibum;

Safek Kidushin is when he threw Kidushin money to her, and we are unsure if it landed closer to him or to her;

Safek divorce is a Get in the husband's handwriting without witnesses, or it has witnesses but no date, or it has a date but only one witness.

(Gemara) Question: Why doesn't the Mishnah list among Safek Gerushin when he threw a Get to her and we are unsure to whom it is closer?

Answer #1 (Rabah): The Tzarah's Chazakah was to be exempt from Yibum or Chalitzah (when her husband will die). When in Safek, we follow her Chazakah. (More answers will be given on Daf 31b.)

Objection (Abaye): If so, we should say the same about Safek Kidushin, (e.g. Leah is Safek Mekudeshes to Reuven)!

The Chazakah of her Tzarah (Reuven's other wife) was to be permitted to do Yibum. We should not forbid her due to Safek!

Answer: There, we are stringent.

Objection: This stringency will lead to a leniency!

If later Reuven was Mekadesh Leah's sister, or another man was Mekadesh Leah (and Reuven died afterwards), since Leah's Tzarah was forbidden to do Yibum, people will think that Leah was Vadai Mekudeshes, so the second Kidushin was invalid (and Leah's sister may marry Reuven's relatives/Leah is permitted)!

Answer: Since we require the Tzarah to do Chalitzah, people realize that this is only a stringency, so they will not err.