אין עונה אמן אחר ברכותיו אלא אחר ב' ברכות או יותר שהם סוף ברכות ונהגו לענות אמן אחר יהללוך ואחר ישתבח: הגה ויש אומרים שאין עונין אמן רק אחר ברכת בונה ירושלים בברכת המזון וכן המנהג פשוט במדינות אלו ואין לשנות (תוס' ומרדכי ר"פ שלשה שאכלו) (מהרי"ק שורש ע"ב) ובמקומות שנהגו לענות אמן אחר יהללוך וישתבח יענה ג"כ אחר ברכת שומר עמו ישראל לעד:

One does not respond "Amen" after his own blessings, except after two or more blessings that conclude a series of blessings. It is customary to respond "Amen" after "Yehallelucha" and after "Yishtabach." Rema: There are those who say that one should respond "Amen" only after the blessing "Boneh Yerushalayim" in Birkat HaMazon, and this is the widespread custom in these countries, and it should not be changed (Tosafot and Mordechai, beginning of the chapter Three Who Ate; Maharik, Root 72). In places where they are accustomed to respond "Amen" after "Yehallelucha" and "Yishtabach", one should also respond after the blessing "Shomer Amo Yisrael La'ad."

If one hears a fellow Jew reciting one of the blessings — even if he did not hear the entire blessing from beginning to end, and even if he himself is not obligated in that blessing — he is required to respond "Amen" after it. However, if the one reciting the blessing was a heretic, or a Samaritan, or a young child, or an adult who altered the established wording of the blessings, one does not respond "Amen" after him. Rema: One may respond "Amen" after a non-Jew if he heard the entire blessing from his mouth (Rabbenu Yitzchak, chapter Eilu Devarim).

The rule that one does not respond "Amen" after a child applies specifically when the child is learning the blessings from his teacher, for it is permitted to teach children the blessings in their proper form even though, during the learning, they are reciting the blessings in vain. However, when the children are reciting the blessings to fulfill their own obligation — since they are of an age of education (bnei chinuch) — one does respond "Amen" after them. The same applies when they recite the haftarah in the synagogue.

Anyone who recites a blessing that is unnecessary is considered as one who takes the Name of Heaven in vain, and it is as if he swore a false oath. It is forbidden to respond "Amen" after him.