Deuteronomy 21:13

and put off the dress of her captivity, and, dipping herself, become a proselyte in thy house, and weep on account of the idols of the house of her father and mother. And thou shalt wait three months to know whether she be with child; and afterwards thou mayest go to her, endow her, and make her thy wife.

she will remove her raiment of captivity perhaps he desired her on account of her beautiful clothing.

weep in order to drive away the joy he has in her.

a full month during which his desire may abate. Alternately, some say that “she will shave” [: 12] because she was formerly ritually impure (as in the treatment of someone afflicted with ṣara‘at [Leviticus 14: 8]).

“Do” [: 12] means rectify , the improvement being that she cut off the excess part (as in “he had not done his beard” [II Samuel 19: 25] and “instead of well-done hair” [Isaiah 3: 24]). The concept of improvement underlies all Scriptural instances of “doing”, except those that refer to disgusting, or completely evil, things.

She will remove her raiment because it is befouled.

The sense of she will stay in your house, and weep is that she must stay in your house a full month while she bewails

her father and her mother, if they were recently deceased. Some say that she weeps for her father and mother because they did not convert to Judaism; but in my opinion, every mentally balanced human being is obliged to honor his father and mother, in life and in death.

weep i.e., she must mourn for them (in the Jewish manner, because she has converted to Judaism) if they were killed at the time she was taken captive. It is unnecessary to mention here that she must immerse herself, since she requires the standard “waters of separation” [Palestinian Talmud Pesaḥim 8: 8 (36b)], as was previously stated: “both yourselves, and your captives” [Numbers 31: 19].

Our Sages, of blessed memory, ruled that one must abstain for three months [Yevamot 48b]. Although there is no need to bring support for their words, the evidence for their reasoning is seen in the verse “about three months later” [Genesis 38: 24] — that being the time when the fetus first begins to stir. It is forbidden to initiate sexual relations with a woman unless one knows whether or not she is pregnant, in order that the paternity of the child may be known — for some babies are born two months earlier than normal, or later than normal (though the latter is less common).

a full month [literally: a lunation of days] as in “a month of days” [Numbers 11: 21]. I have already explained how the term “lunation” denotes a month, since the basis of a month is the lunar cycle.

AND AFTER THAT THOU MAYEST COME UNTO HER, AND BE HER HUSBAND. The Rabbis have commented in the Sifre: 213 and Yebamoth 48b. “You have no privilege in her except cohabitation.” By this the Rabbis meant to say that betrothal with money or a writ is not binding on her. Even the cohabitation is not for the purpose of betrothal, since her status is still that of an idolatress upon whom betrothal is not binding; however, Scripture permitted him to cohabit with her. And because Scripture states [at the expiration of the thirty-day mourning period], and she shall be thy wife, it indicates that she becomes his real wife, and if she commits adultery while living with him, she is to be tried as a married woman. Thus the Rabbis have said in the Gemara of Tractate Kiddushin: “And thou wouldest take her to thee to wife. You have possession of her [through legal marriage],” that is, after following these regulations. And in the Sifre the Rabbis have said, 213 and Yebamoth 48b. “And she shall be thy wife, similar to what is stated, her food, her raiment, and her conjugal rights, he shall not diminish.” If so, she is his wife in all respects seeing that the Torah gave him possession of her.
And in line with the simple meaning of Scripture it appears that he is prohibited from cohabiting with her at all until after he has gone through this entire procedure, this being the sense of the expression, and after that thou mayest come unto her, and be her husband. And that which Scripture stated, and thou desirest her, and thou wouldest take her to thee to wife, means that he would take her for his wife after [complying with] all these regulations which it will command. Similarly, so that I took her to be my wife [means that Pharaoh took her to his house to be his wife]. However, in the Gemara of Tractate Kiddushin the Rabbis have said that he is permitted to engage in the first cohabitation [even before the procedure prescribed in this section], this being the sense of the expression, and thou wouldest take her to thee to wife. They interpreted the expression, “And thou shalt bring her home to thy house — this teaches that he is not to coerce her during battle.” The Rabbis connected it with the verse above, and thou wouldest take her to thee to wife [stated in Verse 11] meaning, when thou shalt bring her home to thy house [Verse 12]. Thus he takes her captive in battle and brings her to his home and to his city, and he cohabits with her in order to bring his lustful heart under control, and after his first cohabitation she is prohibited to him until he follows through the entire procedure with her. This is the way of the Gemara in many places. But I have seen in the Yerushalmi of Tractate Sanhedrin: “Rabbi Yochanan sent to the Rabbis there [in Babylon], in the Land of Israel during the generation following the death of Rabbi Yehudah Hanasi, redactor of the Mishnah. His contemporary in Babylon was Rav, leader of the Academy of Sura. ‘You say two things in the name of Rav which are not so. You say in the name of Rav, a beautiful woman was permitted only for the first cohabitation [prior to the prescribed procedure].’ And I say neither the first cohabitation nor the second is permitted save only after [complying with] all these regulations, [for it is written], and after that thou mayest come unto her, and be her husband — after she has gone through all these prescribed deeds.” And this is the plain sense of the verse.

והסירה את שמלת שביה, in order to make her appear less attractive.

ובכתה את אביה ואת אמה, to get her father and mother out of her system and reconciling herself to the fact that henceforth she would live in the Land of Israel, have no more contact with her parents. (whether they are dead or alive) Similar wording occurs in Psalms 45,11 שכחי עמך, the psalmist urging the Jewish people, compared to a princess, to forget their former environment and associates as she had been promoted to a loftier position. The prisoner of war described here as embracing Judaism has similarly been promoted to a far superior environment, severing her previous relationships completely. The weeping mentioned in our verse has nothing to do with the death of her parents for we do not kill mothers altogether. She weeps for having no longer a father and mother in the legal sense as these did not convert. Any other convert, not a prisoner of war, would similarly weep for this separation having occurred in her life as a result of her new status.