If one began to say the weekday shemoneh esrei on Shabbos, at ma’ariv, shacharis, or minchah, the person should conclude the beracha, and then continue with the correct shemoneh esrei.
At mussaf , stop immediately and continue with tikanta Shabbos….
If the person only said the word atta (thinking to continue with the word chonein):
- At maariv continue with kidashta.
- At Minchah continue with echad.
- At Shacharis:
- If it was a slip of the tongue, stop immediately and say yismach Moshe.
- If the person absent-mindedly thought it was a weekday, continue ata chonein, finish the beracha, and then say yismach Moshe.
- At Mussaf, stop immediately, and continue with tikanta Shabbos.