76:17 Even if one only said just the first word of a weekday intermediate bracha, then he immediately realized his mistake, he must finish the bracha he started. The sole exception is if one started the bracha “Atah chonein” on Shabbos at Maariv or Mincha. The intermediate Shabbos brachos of those two prayer services also start “Atah,” so if all one said was that one word, he may change gears and say the bracha of “Atah kidashta” or “Atah echad” instead. However, at Shacharis, he must finish the bracha “Atah chonein” that he started. This is the case if he momentarily forgot it was Shabbos and thought it was a weekday. If he knew it was Shabbos and meant to say the bracha “Yismach Moshe” but he started the wrong bracha through force of habit, he does not need to complete the bracha if all he said was “Atah.” The reason for this is because some of the Shabbos prayers also start “Atah,” so it’s as if he made a mistake with parts of the Shabbos prayers.
76:18 If one did not realize his mistake until he reached the last three brachos, namely from R’tzei until the end, then he should stop in the middle of the bracha where he realized and begin the proper intermediate bracha for Shabbos or yom tov and continue in order from there. If he only realized after he started to say “Yihiyu l’ratzon” (which concludes the Amidah – Mishnah Brurah 117:18), he must go back to the beginning and repeat the Amidah all over again. (If one doesn’t know whether he said the Shabbos bracha or a weekday bracha, he must repeat because he is more likely to have said the weekday text through force of habit – MB 268:9.)