On Rosh Hashana, the third Brocho of Shmoneh Esrei has a long addition beginning with the words L’Dor V’dor. If one omitted this addition, he may go back to recite it only if he realised before saying Hashem’s name at the end of the third Brocho. Otherwise, he continues Shmoneh Esrei without going back, as long as he is sure that he concluded the Brocho with the words Hamelech Hakodosh.
If one recited the unique Rosh Hashana Nusach of the third Brocho, but is in doubt whether he concluded the Brocho with the words Hamelech Hakodosh, he may assume that he did so correctly.
One who did not say Hamelech Hakodosh: If he realized before he began the next Brocho and within the time frame it takes to say three words, he corrects his mistake. Otherwise, he must begin Shmoneh Esrei again. [The same applies if this occurs in Chazaras Hashatz, in which case Kedushah is recited again.]
If one mistakenly omitted any of the other four additions (Zochreinu, Mi Chomocha, u’Chsov, u’Vsefer Chaim), he may recite it at the place he remembers if he has not yet said Hashem’s name at the end of that Brocho. Otherwise, he continues Shmoneh Esrei and does not go back.
During Chazaras Hashatz, the congregation recites the selections of U’chsov and u’Vsefer Chaim aloud before the Chazzan.
