On Yom Kippur, the third Brachah of the Amidah 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 realized before saying Hashem’s name at the end of the third Brachah.
Otherwise, he continues the Amidah without going back, as long as he is sure that he concluded the Brachah with the words Hamelech Hakadosh.
If one recited the unique Yom Kippur Nusach of the third Brachah, but is in doubt whether he concluded the Brachah with the words Hamelech Hakadosh, he may assume that he did so correctly.
One who did not say Hamelech Hakadosh: If he realized before he began the next Brachah and within the time frame it takes to say three words, he corrects his mistake. Otherwise, he must begin the Amidah again.
If one mistakenly omitted any of the other four additions (Zachreinu, Mi Chamocha, u’Chtov, 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 Brachah. Otherwise, he continues the Amidah & does not go back.
During the repetition of Amidah, some hold the congregation recites the selections of u’Ktov and u’Vsefer Chaim aloud before the Chazzan.
