What made Yehudah assume that she was a Zonah?
Rashi #1: Because otherwise, what she was doing sitting at the cross-roads?
Malbim: In lands of Yishma'el, the custom is to burn a letter on the face of harlots, so people will know that she is a harlot. They cover their faces so the letter will not be seen. Later, Yehudah commanded to burn [a letter on the face of] Tamar.
Why did he not recognize her?
Rashi #1 and Targum Onkelos: Because her face was covered [now]. 1
Rashi #2 and Targum Yonasan: Because when she lived in his house, she had always covered her face, so there was no way that he could have known who she was. 2
Seforno: Refer to 38:16:1:1.
See Ramban, who elaborates on this explanation. Ha'amek Davar - Because he did not recognize her from afar, and assumed that she is a Zonah, he did not recognize her through her voice.
Also refer to 38:15:2.1:2.
QUESTIONS ON RASHI
Rashi writes: "'For she covered her face' - ... Our Rabbis interpret, for she had covered her face while in her father-in law's house; she was modest...." Why does Rashi cite the Midrash?
Gur Aryeh: Were it to mean only that she had covered her face now, we already know this from the word "va'Tis'alaf" (38:14), which Rashi (ibid.) interpreted to mean that she concealed her face.
Sifsei Chachamim, Oznayim la'Torah: Even if her face was covered initially, he should have recognized her as Tamar upon speaking to her, and she surely uncovered her face prior to relations. 1 Rather, because Tamar had been so modest in his home, he did not suspect that this was actually her.
Rashi writes: "... For she had covered her face while in her father-in law's house; she was modest." What did Tamar merit as a result of her modesty?
Maharal (Chidushei Agados Vol. 2, p. 42, to Sotah 10b): The Gemara says that Tamar merited to be the ancestress of kings and prophets. Modesty is sanctifying oneself against indecency, a trait that transcends the physical. So too would her descendants merit G-dly distinction.
