the word yasaf is frequently difficult. sometimes it means did not stop and sometimes it means did not continue. if here it means did not stop, then how did yehuda continue to have relations with tamar, wasn't she assur to him?
dmartin, raanana eretz yisroel
Your question is that she was Asur to him as his daughter-in-law (who is Asur to her father-in-law even after the death of his son, her husband). The Gemara says that she made sure that she was permitted to him by pointing out that she was a Yesomah. Since she was a Yesomah, her father did not accept Kidushin for her, and thus her Kidushin was not mid'Oraisa. (This answer to your question applies only according to our Gemara that says that Tamar was a Ketanah at the time of her marriage to Yehudah's son. According to the Midrash that says she was a Gedolah, perhaps "Lo Yasaf" means that he did not continue to live with her.)
M. Kornfeld
Please note that the Ramban tells us that in actuality Yehuda did Yibum with Tamar since before Matan Torah the Mitzvah of Yibum was done by other relatives and not only a brother-in-law. Thus the Mitzvah of Yibum was being done by Yehauda although Tamar was his daughter-in-law. Therefore she was not forbidden to him.
Rabbi Yosef Grossman Rosh Kollel Daf Yomi - Ohr HaMeir, Monsey, NY.
Thank you for your beautiful explanation.
(As for our Gemara, in which Tamar has to excuse away her marriage to Er and Onan by saying she was a Yesomah, our Gemara might be following the opinion of Abba Shaul, that Yibum must be done Lishmah and not for any ulterior motive, Yevamos 39b. Since Yehudah did not know it was a Yibum, it was only permitted for him since she was not really his daughter in law. See also Hagahos Ya'avetz here, who explains how our Gemara explains the words "v'Yabem Osah" in the Pasuk, if she was not really married to Er and Onan.)
-M. Kornfeld