If one forgets Ya’aleh Veyavo in davening, but remembers before saying Hashem’s name at the end of Hamachazir Shechinasoi L’tziyon, he goes back. If he remembered between the conclusion of that Brocho and Modim, he recites it at that point, without going back. If he remembers any time after that point, but before the second Yih’yu L’ratzon (at the end of Elokai N’tzor), he returns to the beginning of Retzei. If he remembers after that, Shmoneh Esrei is repeated.
If one is unsure whether he recited Ya’aleh Veyavo, Shmoneh Esrei is repeated.
If one forgot Ya’aleh Veyavo during Shacharis, and only realised after Musaf, he does not repeat Shacharis.
If one forgot Ya’aleh Veyavo during Mincha or Maariv, and only realized after the Zman Tefillah has passed, he must recite an additional Shmoneh Esrei in the next Tefillah, as compensation. Between the two Tefillos, he should wait the span of time to walk four Amos (approx. two metres), and preferably, he should recite words of supplication between them.
