On what grounds does the Torah permit marrying an Eishes Y'fas To'ar?
Why does the Torah insert the word "Eishes"?
Rashi: To permit her even if she is married.
What if she is not beautiful?
Ramban (citing the Sifri): Her beauty is irrelevant. 1
Why does the Torah insert the words "Vechashakta bah"?
Ramban (citing the Sifri): To teach us a. that the Heter is based on his desire for her, and that, consequently, she is permitted to him even if she is not beautiful and b. that he is only permitted if he has a desire to be intimate with her, but not if he merely wants a wife.
What are the connotations of "Velakachta l'cha le'Ishah"?
Rashi: Refer to 21:13:5:2*.
Ramban (citing Kidushin, 21b and Sifri): It means that the first Bi'ah is permitted immediately. 1
Riva: This teaches that you intend to take her for a wife. Below (verse 13), it says "u'Ve'altah v'Haysah Lecha l'Ishah." This implies that before that, she is not your wife.
Why does the Torah add the word "L'cha"?
Ramban (citing the Sifri): To teach us that he is not permitted to take her as a wife for his father or his brother. 1
Ramban (citing Kidushin, 22b): Or to take two women, one for his son and one for himself (See Ramban).
Why does the Torah say Ye'fas To'ar, and not 'Yefas Mar'eh'?
Rosh: They were under siege, and due to hunger, their faces change [detrimentally]. It says Ye'fas To'ar, for she is proper to receive the beauty 1 that was lost in the days of hunger.
QUESTIONS ON RASHI
Rashi writes that "Eishes" permits even if she is married. "Eshes Re'ehu" already excludes [from the Isur] a Nochri's wife (refer to Vayikra 20:10:3:1)!
Riva citing R. Elyakim, from Sanhedrin 57a: If not for the Drashah here, it would be forbidden due to theft (taking another man's wife 1 ), but not due to Arayos.
Riva: The Drashah here is based on the word Eshes in Eshes Re'ehu.
Riva: Some say that the Drashah of Eshes Re'ehu excludes only Misah, but there is a Lav. Here, the Torah totally permitted it. 2
Riva citing R. Tam, Moshav Zekenim: If not for the verse, we would have said that there is an Isur Asei "v'Davak b'Ishto" (Bereishis 2:24), and not to another's wife. The verse teaches that there is not even an Isur Asei.
If we may kill her husband in war, is it forbidden to take his property?! Perhaps the Heter is needed for a captive whose husband is from a nation not involved in the war. (PF)
Riva: This is wrong. Chachamim decreed NaShGaZ (one who has Bi'ah with a Nochris, it is as if he had Bi'ah with a Nidah, Shifchah, Goyah and Zonah (Avodah Zarah 36b. It seems that the text should say NaShGA, like the opinion there that includes Eshes Ish
Rashi writes that the Torah permits her to counter the Yetzer-ha'Ra. In Kidushin (22a), Rashi writes that Bi'ah is forbidden until after the entire Parshah (including a month of mourning
Moshav Zekenim citing R. Tam: One Bi'ah is permitted immediately. The Gemara forbids a second Bi'ah before all the actions.
Moshav Zekenim: Because he knows that she will be permitted afterwards, immediately the Yetzer ha'Ra for her is smaller (and he can control himself until she is permitted).