Atta Chonatanu

1. On Saturday night (Motzei Shabbat) in the fourth Bracha of Shemonah Esrei (of Arvit) one should add in the paragraph of Atta Chonatanu which serves partially as Havdalah.
2. On Motzei Yom Tov (night of the conclusion of Yom Tov) one should say Atta Chonantanu in Shemonah Esrei of Arvit/Mariv even if the Yom Tov is following by Chol HaMoed.

If one forgot Atta Chonatanu

1. If one forgot to say Havdalah in Shemonah Esrei (Atta Chonatanu in Atta Chonen, the fourth Bracha) once one finished the Bracha even if one didn’t begin the next Bracha one shouldn’t go back to say Atta Chonatanu.
2. If one forgot Atta Chonatanu and finished Shemonah Esrei one doesn’t have to repeat Shemonah Esrei, however, if one wanted to be strict on oneself one may repeat it. However, one not be strict upon oneself after having finished the Bracha to go back to the Bracha in order to add Atta Chonatanu.

Cases one has to go back and say Atta Chonatanu

1. If one forgot Atta Chonatanu and then mistakenly ate before making Havdalah one must repeat Shemonah Esrei with Atta Chonatanu.
2. If one forgot Atta Chonatanu and one knows (thinks) that he will not be able to make Havdalah neither Saturday night nor Sunday then one should repeat Shemonah Esrei to add in Atta Chonatanu.

VeTodienu

1. When Yom Tov falls out on Motzei Shabbat (Saturday night) one is supposed to say VeTodienu in the Arvit/Mariv Shemonah Esrei.
2. If one forgot to say VeTodienu one doesn’t have to repeat the Bracha or Shemonah Esrei like Atta Chonatanu. However, if one ate before making Kiddish one has to repeat Shemonah Esrei (like Atta Chonatanu).