"of beautiful form": This tells me only of a woman of beautiful form. Whence do I derive that the same applies to an ugly woman? From "and you desire her": even if she is not beautiful. If so, why is it written of beautiful form"? Scripture speaks of the common instance.
(דברים כא י) כִּי תֵצֵא לַמִּלְחָמָה – בְּמִלְחֶמֶת הָרְשׁוּת הַכָּתוּב מְדַבֵּר.
(Devarim 21:10) "If you go out to war": Scripture speaks of an optional war (as opposed to a war of mitzvah).
עַל אֹיְבֶיךָ – כְּנֶגֶד אוֹיְבֶיךָ.
"al your foes": against your foes.
וּנְתָנוֹ ה׳ אֱלֹהֶיךָ בְּיָדֶךָ, אִם עָשִׂיתָ כָּל הָאָמוּר בָּעִנְיָן – סוֹף שֶׁה׳ אֱלֹהֶיךָ נוֹתְנוֹ בְּיָדֶךָ.
"and the L-rd your G-d delivers him into your hand": If you do as stated herein, in the end, the L-rd your G-d will deliver him into your hand.
וְשָׁבִיתָ שִׁבְיוֹ – לְרַבּוֹת כְּנַעֲנִים שֶׁבְּתוֹכָהּ.
"and you capture its captivity": including Canaanites in their midst (even if they be of the seven nations).
(דברים כא יא) וְרָאִיתָ בַּשִּׁבְיָה – בִּשְׁעַת שִׁבְיָה.
(Ibid. 11) "and you see in the captivity": at the time of the captivity.
אֵשֶׁת – וְאַף עַל פִּי שֶׁהִיא אֵשֶׁת אִישׁ.
"a woman": even a married woman.
יְפַת תֹּאַר, אֵין לִי אֶלָּא בִּזְמַן שֶׁהִיא נָאָה, מִנַּיִן אֲפִילּוּ הִיא כְּעוּרָה? – תַּלְמוּד לוֹמַר: וְחָשַׁקְתָּ בָהּ, אַף עַל פִּי שֶׁאֵינָהּ יְפַת תֹּאַר.
"of beautiful form": This tells me only of a woman of beautiful form. Whence do I derive that the same applies to an ugly woman? From "and you desire her": even if she is not beautiful. If so, why is it written of beautiful form"? Scripture speaks of the common instance.
וְלָקַחְתָּ לְךָ לְאִשָּׁה, שֶׁלֹּא תֹּאמַר ׳הֲרֵי זוֹ לְאַבָּה׳, ׳הֲרֵי זוֹ לְאָחִי׳.
"then you may take her for yourself as a wife": and not for his father and not for his son.