If one forgets to say “ata chonantanu” at ma’ariv on Motza’ei Shabbbos:
- if the person already said the shem HaShem at the conclusion of the beracha, the person should continue shemoneh esrei, and say ata chonantanu before (the second) yiheyu l’ratzon at the conclusion of shemoneh esrei.
- If the person didn’t say it there either, the person cannot do any melachah until saying baruch hamavdil bein kodesh lechol, or until saying (or hearing) Havdalah.
There is an interesting halacha as follows: Normally, if one ate or drank before havdalah, although one is not allowed to (with the exception of water), there is no penalty. BUT, if one forgot ata chonantanu AND ALSO–even mistakenly—ate or drank (something besides water) before havdalah, there is a penalty, and the person must repeat shemonah esrei (with ata chonantanu) before Havdalah.