Havdala is recited twice, the first time in Maariv with the blessing of Ata Chonantanuand again over a cup of wine.
Why do we say Havdala in the blessing of Atah Chonen? The Yerushalmi in Brachot says, “Im ein da’at havdala minayin.” Without intellect, it is impossible to distinguish between different things. To make Havdala, one needs a measure of intellect. On Shabbat we are focused solely on spirituality. We don’t ask for any physical needs. Havdala serves as a sanction to begin working again. Therefore, Ata Chonantanu is inserted before the first supplication of mercy in order to allow us to engage in further requests.
If you forget Ata Chonantanu, you don’t have to recite it again, if you will be reciting Havdala over wine. If you didn’t say either of them, you must recite the prayer in Shachrit. The Mishna Berura cites the Magen Avraham that the makeup prayer would be the second prayer. Rav Akiva Eiger disagrees. Ata Chonantanu should be inserted in the first prayer because one needs its sanction to engage in further supplications.
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