Yevamos, 52b: To teach us that a Yavam who divorces his Yevamah is not permitted to perform Yibum with her, even though she did not marry in between. ¹

Kidushin, 75a: "Lashuv Lekachtah" implies that Machzir Gerushaso is only Chayav if he betrothes her, but not if is merely intimate with her, ¹ whereas "Lih'yos lo le'Ishah" implies that he is only Chayav if he is also intimate with her, but not if he merely betroths her. ²

Rashi: To include a Sotah who was secluded (with whom she had relations - Ramban) with the man she was warned about in the prohibition. ¹

Ramban #1: The Pasuk is presenting two La'avin here, 1. Against a husband taking back his wife after she commited adultery and he sent her away; 2. Against a husband taking back his divorced wife after she was married to another man.

Ramban #2: The Torah means simply that now that she became Tamei (forbidden) to her first husband - by virtue of her having had relations with another man (her second husband - albeit be'Heter) she is forbidden to return to him. ²

Yevamos 56b: If a married woman commited adultery, "Acharei asher Hutama'ah" forbids her to her husband. ³ In the event that she was an Oneis, the Torah wrote - "ve'Hi Lo Nispasah" permitting the wife of a Yisrael. But the wife of a Kohen remains forbidden to her husband.

Seforno: It is referring to the disgusting habit of wife-swapping - where a man divorces his wife so that another man may sleep with her before her husband remarries her. ¹

Oznaim la'Torah (citing Sefer ha'Chinuch): Going back to her first husband after having lived with other men is a form of adultery.

Targum Yonasan and Yevamos, 11b: "Hi" To'evah, ve'Ein Banehah To'evim' - to render Kasher the children of a divorced woman whose husband took her back after she was married to another man. ¹

Ramban and Seforno: To avoid wife-swapping - where (at the request of Shimon - Seforno) Reuven divorces his wife in the evening, Shimon marries her and lives with her for one night and divorces her, and Reuven takes her back in the morning.

Moshav Zekenim: If not for this Isur, the La'av of coveting would be nullified. A rich man who desires a poor man's wife would tell him 'divorce your wife and I will marry her for a month or two, and divorced her and return her ¹ .'

The Gemara in Yevamos 108b states that if a girl performs Mi'un, she may not return to a man who previously divorced her. If it were permitted, he might gesture and wink at her to do Mi'un to her new husband, in order to remarry him. Perhaps this concern applies even to Torah marriage. She cannot divorce herself, but she can anger her husband until he will want to divorce her. ² (PF)

Yerushalmi Gitin, 9:11: Because a Sotah is forbidden to her husband only with an Asei, and the Torah is coming here to add a Lo Sa'aseh. ¹

Seforno: Refer to 24:4:2:1.

Targum Yonasan and Yevamos 11b: It implies that although she is an abomination, the children that she will bear are Kasher ¹ (and are even eligible to marry a Kohen Gadol - Nidah, 70a).

Ramban: Because of the major sins to which this leads. ¹

Sifri: It is a warning to Beis-Din to ensure that men do not transgress the La'av of Machzir Gerushaso. ²

Targum Yonasan: It is a warning that the Isur Machzir Gerushaso causes the land to die (not to produce its frui). ³

Kidushin 78a: Machzir Gerushaso receives Malkos only if he was intimate with her, but not for Kidushin alone, which does not fall under the category of "Lih'yos lo le'Ishah".

This is not difficult according to the Ramban (Hilchos Melachim 4:4), who permits a Pilegesh only to a king, or an Amah Ivriyah after Yi'ud - and the Torah cannot be discussing a king's wife, since she is forbidden to others even after divorce, and everyone agrees that her father cannot sell her to be a slave after marriage (Kidushin 18a). ¹ (PF)..

Ha'Kesav veha'Kabalah: It is a graft of Huf'al (which would be 'Hatme'ah') and Hispa'el (which would be 'Hasta'm'ah', or 'Hateme'ah' with a Dagesh on the 'Tes'). There are three cases, with different reasons to forbid her: 1. If her first husband divorced her due to Z'nus be'Mezid, this itself forbids her to him permanently. This corresponding to Hispa'el, which implies Ratzon; 2. If he divorced her because he thought erroneously that she commited adultery, the divorce was invalid, and her second 'marriage' was Z'nus with an Eishes Ish, which forbids her to her real husband. The Tum'ah was Oneis, since she married legally based on a Get, This corresponding to Huf'al, which implies coerced; 3. If he divorced her absolutely (without any stipulation), or not due to adultery, it is a Gezeiras ha'Kasuv that she is called Tamei and forbidden to return. Rabanan forbid her even if the latter was merely Mekadesh her (refer to 24:4:153:1,2)! The Torah used an expression of Tum'ah to teach about one who commited adultery under her husband. ¹

Meshech Chachmah: It is Huf'al, which implies coerced ? "v'Yosef Hurad" (Bereishis 39:1). Whenever it says "Nitma" (conjugation Nif'al), the person caused it to himself. Here, a Kohen's wife is forbidden even b'Ones.

Yevamos 11b #1 (according to R. Yossi ben Kipar): It is only if she had married her second husband - "Acharei asher Hutama'ah".

Yevamos 11b #2 (according to the Chachamim): The Isur applies irrespective as to whether the second husband betrothed her or married her ¹ - because "Acharei asher Hutama'ah" is referring to a Sotah. ²