(Mishnah): In all these cases (when Reuven may not marry Leah because he permitted her to marry), if he had a wife and she died, he may then marry Leah;

If Leah married someone else and was widowed or divorced, Reuven may then marry her;

In all cases, Leah is permitted to Reuven's children and brothers.

(Gemara) Inference: This is only if Reuven's wife died, but not if he divorced her.

Contradiction (Rav Hillel - Beraisa): It is even if he divorced her.

Answer #1 (Rav Ashi): Reuven may marry Leah only if he had previously quarreled with his wife (Rashi; Me'iri - only if there is a real quarrel.)

Answer #2: In both cases, there was no previous quarrel (Rashi; Me'iri - there is a quarrel.)

If Reuven (later) started the quarrel, he may not marry Leah. If his wife started it, he may.

(Mishnah): If they married...

Assumption: This is even when a widowed woman remarried and was widowed again, and a divorced woman was divorced again.

Suggestion: Our Mishnah is unlike Rebbi, who says that a Chazakah is established after two occurrences (so a woman who was widowed twice may not remarry).

Rejection: It can be like Rebbi. The Mishnah permits a widowed woman who was later divorced, and a divorced woman that was later widowed.

(Mishnah): They are all permitted to his sons and brothers.

Question (Mishnah): A man suspected of Bi'ah with Rachel may not marry her mother, daughter or sister.

Why is our case different?

Answer #1: Women frequently visit each other (if he marries Rachel's relative, he may come to have Bi'ah with Rachel, and this is Chayavei Kerisus)!

Men do not frequently visit each other.

Answer #2: Women do not forbid each other (if Leah's relative has Bi'ah with Leah's husband, this does not forbid Leah to her husband, therefore a woman is not concerned if her husband has Bi'ah with her relatives;

Men do forbid each other (if Levi's wife has Bi'ah with his relative (or any other man), she becomes forbidden to Levi), so a man distances his wife from his relatives.

Question: If so, the woman should be permitted even to his father!

Answer: She is! The Mishnah teaches a bigger Chidush:

If Reuven permitted a woman, not only his father may marry her, in which case Reuven is ashamed to sin with his father's wife;

Even Reuven's son may marry her. Even though a man is not ashamed to be with his son's wife, we are not concerned.

(Mishnah): If two of four brothers were married to sisters, and these two died, the widows do Chalitzah, not Yibum. If they did Yibum, they must be divorced;

R. Eliezer says, Beis Shamai allow them to stay married. Beis Hillel say that they must be divorced.

If one of the sisters was an Ervah to (e.g.) Reuven, he is forbidden to her and permitted to her sister. Shimon (the other surviving brother) is forbidden to both;

If a sister is forbidden to a brother due to an Isur Mitzvah or Isur Kedushah, she does Chalitzah but not Yibum;

If each sister is Ervah to a different brother, each is permitted to the brother to whom she is not Ervah;

This fulfills 'when her sister is her Yevamah, she does Chalitzah or Yibum.'

(Gemara): Our Mishnah shows that Yesh Zikah.

If not, since the sisters fall from different brothers, each Yavam could do Yibum with one of them!

Rejection: Perhaps Ein Zikah, but it is forbidden to cause that the Mitzvah of Yibum might be lost;

If we allowed Yibum, perhaps by the time Reuven does Yibum, Shimon will die, and the other Yevamah (who is now Reuven's Achos Ishto) will be exempt from Yibum and Chalitzah.

Question: If so, even when (at the beginning) there were three brothers, the sisters should do Chalitzah and not Yibum!

Answer: True! The Tana taught even more.

If there were three brothers, if the surviving brother does Yibum, the Mitzvah of Yibum is perforce Batel regarding her sister.

When there are four brothers, one might have thought that we are not concerned that one may die and the Mitzvah will be lost. The Mishnah teaches that this is not so.

Question: If so, even when there were five brothers, Yibum should not be allowed!

Answer: We are not concerned lest two (of the three surviving) brothers die.

(Rav): If three sisters fell to Yibum to two brothers, one sister does Chalitzah with one brother, another does Chalitzah with the other brother, and the third sister needs Chalitzah from both of them.

(Rabah): Since Rav requires a sister to do Chalitzah with both brothers, he must hold that Yesh Zikah, so the third sister's Chalitzah is Pesulah (Rebbi - since Yibum was forbidden; Tosfos - since the Zikah is weakened), and one who did Chalitzah Pesulah must do Chalitzah with all the brothers.

Question: If so, also the first two sisters should need Chalitzah from all the brothers!

Answer: If they fell together (the second fell before the first did Chalitzah), that would be true. The case is, each fell after the previous one (did Chalitzah).

One sister fell, and Reuven did Chalitzah with her. Another sister fell, and Shimon did Chalitzah with her. When the third falls, each does Chalitzah to remove his Zikah to her.

Question: Rav (17b) taught that Ein Zikah!

Answer: Here he teaches according to the opinion that Yesh Zikah.