In some synagogues, it is customary for the Hazan on Mosa’eh Shabbat to recite "Ata Honantanu" in a loud voice during the Amida, in order to remind the congregation to insert this paragraph. The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) writes that long ago, before Siddurim were widely accessible, the Hazan would recite the entire text of "Ata Honantanu" out loud so the congregants could either listen or recite the text along with the Hazan. Today, of course, we use printed Siddurim, but we are prone to forgetting to insert this recitation. One must exercise care to remember to recite "Ata Honantanu" in the Amida on Mosa’eh Shabbat.
If one forgets to recite "Ata Honantanu," but he realizes his mistake before reciting "Baruch Ata Hashem," then he goes back to recite "Ata Honantanu." If he realized his mistake only after reciting the words "Baruch Ata Hashem," then he continues and does not go back to recite "Ata Honantanu." Although some Halachic authorities maintained that one does go back even in this case, the Shulhan Aruch ruled that one simply continues the Amida without reciting "Ata Honantanu."
The Aruch Ha’shulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908) cites those who maintain that if one forgot to add "Ata Honantanu" in the place where it should be recited, he has the option of adding it later in the Amida, in the Beracha of "Shome’a Tefila," where one is allowed to add any personal requests he has. Hacham Ovadia Yosef, however, notes that in "Shome’a Tefila" one may add only requests, whereas "Ata Honantanu" is a declaration affirming that G-d has distinguished between sacred and mundane, Shabbat and the weekdays, and so on. Therefore, it would be inappropriate to recite "Ata Honantanu" in "Shema Kolenu." If one wishes to add "Ata Honantanu" in the Amida, Hacham Ovadia writes, he may add it at the very end, after reciting the verse, "Yiheyu Le’rason Imreh Fi…"
If one forgot to recite "Ata Honantanu" in the Amida on Mosa’eh Shabbat, and then ate before reciting or hearing Habdala, then he must repeat the Amida with "Ata Honantanu." The Sages imposed a "penalty" upon one who made both these mistakes – omitting "Ata Honantanu" and eating before Habdala – requiring him to repeat the Amida. Hacham Bension Abba Shaul (Israel, 1923-1998), in his Or Le’sion (1:22), addresses the question of whether this also applies to somebody who forgot to add "Ata Honantanu" and then performed Melacha (activity forbidden on Shabbat) before Habdala. As such a person also made two mistakes, perhaps he, too, should be required to repeat the Amida. After a lengthy discussion, Hacham Bension concludes that a person in this case would not be required to repeat the Amida, since performing Melacha before reciting "Baruch Ha’mabdil Ben Kodesh Le’hol" is not considered a serious Halachic breach. While one should certainly refrain from Melacha before reciting "Baruch Ha’mabdil," such a mistake would not warrant the same "penalty" imposed on somebody who ate before Habdala.
Hacham Bension further notes that the requirement to repeat the Amida applies if one ate a "Ke’zayit" of food or drank a "Rebi’it" of beverage before Habdala. Eating or drinking less than these amounts does not require one to repeat the Amida.
If a person forgot or was unable to recite Arbit on Mosa’eh Shabbat, and must therefore recite an extra Amida the next morning at Shaharit, must he add "Ata Honantanu" in one of the two Amida prayers? Most likely, a person in this situation recited or heard Habdala at some point on Mosa’eh Shabbat, or plans on reciting Habdala later on Sunday. Therefore, he does not recite "Ata Honantanu." If, however, he had not recited Habdala and does not plan on reciting Habdala, because he does not have access to wine, then he must recite "Ata Honantanu." Hacham David Yosef, in his Halacha Berura, writes that a person in this case recites "Ata Honantanu" in the second Amida which he recites on Sunday morning, as this is the prayer which he recites to make up for the missed Arbit prayer. Nevertheless, he may, if he wishes, recite "Ata Honantanu" in the first Amida, following the ruling of Rabbi Akiva Eger (1761-1837).
Summary: One who forgot to recite "Ata Honantanu" in the Amida on Mosa’eh Shabbat, and did not realize his mistake until after reciting the words "Baruch Ata Hashem," does not repeat the Amida. If he wishes, he may insert "Ata Honantanu" at the very end of the Amida, after the verse, "Yiheyu Le’rason Imreh Fi." If one forgot to recite "Ata Honantanu" and then ate a "Ke’zayit" or drank a "Rebi’it" before Habdala, he must repeat the Amida with "Ata Honantanu." If one forgot or was unable to recite Arbit on Mosa’eh Shabbat, and must therefore recite an extra Amida the next morning at Shaharit, he does not recite "Ata Honantanu," unless he had not recited Habdala and will not be able to recite Habdala, in which case he recites "Ata Honantanu" in the second Amida.
