Siman 215

One who hears a fellow Jew reciting any berakhah, even if he did not hear the entire berakhah but only its conclusion, is obligated to answer "amen," even if he is not obligated in this berakhah. And it happened with one of the great ones who was studying with his teacher, and a child ate a fruit and recited a berakhah over it aloud, and this great one did not answer "amen," and his teacher rebuked him with a great rebuke as is explained in Yoreh Deah siman 334, and he conducted himself with rebuke and afterward appeased him and forgave him on the condition that he preach to the whole world how great is the punishment for one who hears a berakhah and does not answer "amen," and he told him a fearsome story regarding this matter (Levush in the introduction). Aside from this, also when one hears them performing a "Mi Shebeirakh" for someone, it is proper to answer "amen," and in this he fulfills the commandment of "and you shall love your fellow as yourself," and therefore we answer "amen" after the Harachaman (Magen Avraham subsection 3).

But if the one reciting the blessing was a Kuti who worships Mount Gerizim, or an apikorus, or a child who does not know at all what he is saying, or if he was an adult and deviated from the established formula of the blessings—one does not answer 'amen' after him. Similarly, one does not answer 'amen' after a child while he is learning the blessings before his teacher, for it is permitted to teach children the blessings in their proper form even though they are reciting blessings in vain during the time of study. However, an adult during his study should not say the Name explicitly, but rather should say 'Hashem', and similarly he should say 'Elokeinu'. And so too those who lecture in public and mention a verse that contains Names should not articulate the Name but should say 'Hashem'; and even though there are those who permit it, it is proper to distance oneself from this, and so we practice. But a child who recites a blessing to fulfill his own obligation and understands what he is saying and has reached the age of chinuch—one answers 'amen' after him; and similarly after a child who recites the Maftir in the synagogue—one answers 'amen' after him. And it appears to me that regarding a female child who is not of the age of chinuch, one does not answer 'amen' after her until she is twelve years and one day old.

Our teacher, the Rema, wrote in section 2: "And one answers after a non-Jew if he heard the entire blessing from his mouth." Thus far his words. And it is true that something similar to this is found in the Yerushalmi, but there are those who disagree, and so it is explained from the words of the Rambam in chapter 1 and from his commentary on the Mishnayot in chapter 8, see there; and the Yerushalmi is at the end of chapter 8 of Berakhot. And it appears in my humble opinion that his intention is not regarding a standard, established blessing, but rather that he said a blessing to the God of Israel or the like, see there.

One does not answer 'amen' except after the blessing of another and not after one's own blessing, and one who answers 'amen' after his own blessings, this is disgraceful (Gemara 45b). And it appears to my humble opinion that the reason is that behold, through the blessing, supernal abundance is influenced, and the answering of 'amen' is to strengthen the matter that it is certainly so, and it is not fitting that a person should strengthen himself as if he is worthy that through him the abundance of the blessing should flow, and therefore it is disgraceful, and another may say so but not himself. And only after the conclusion of a matter of two or three blessings may one answer 'amen' at its end, as we answer in Birkat HaMazon after 'Boneh Yerushalayim' which is the conclusion of the blessings from the Torah, since 'HaTov VeHaMeitiv' is not from the Torah, and they did not practice so with other types of blessings. Our teacher, the Beit Yosef, wrote that they practiced answering 'amen' after 'Yehalelukha' and after 'Yishtabach', see there, and our teacher, the Rema, wrote regarding this: "And there are those who say that one does not answer 'amen' except after the blessing of 'Boneh Yerushalayim' in Birkat HaMazon, and such is the simple custom in these lands and one should not change it. And in places where they practiced answering 'amen' after 'Yehalelukha' and 'Yishtabach', he should also answer after the blessing of 'Shomer Amo Yisrael La'ad'." Thus far his words. And consequently, on Shabbat and Yom Tov he should answer after 'HaPores Sukkat Shalom', but among us the custom is only in Birkat HaMazon at 'Boneh Yerushalayim', and perhaps in order to make a distinction between a Torah commandment and a rabbinic one, and therefore they did not practice so in other blessings because this is not an obligation but a permission, for at the conclusion it is permitted to answer 'amen' after one's own blessing but not an obligation.

Anyone who recites a blessing that is not needed—behold, he takes the Name of Heaven in vain, and he is like one who swears in vain, and it is forbidden to answer "amen" after him. And even though this is not a literal Torah prohibition like swearing in vain or uttering the Name of Heaven for no purpose, which refers to mentioning the Name without a blessing, whereas in a blessing the prohibition is rabbinic (Tosafot at the end of Rosh Hashanah and the Rosh in Kiddushin chapter 1), and that which they said in Berakhot (33a) that one who recites an unnecessary blessing violates "you shall not take [the Name of the Lord your God in vain]" is a mere asmachta (ibid.), nevertheless, it is an exceedingly severe prohibition. There is one who wishes to say that the Rambam in chapter 1 of Berakhot, law 15, holds that it is a Torah prohibition (Magen Avraham subsection 6), but it is not definitive (Eliyah Rabbah subsection 5). And there is one who wrote that when one mentions the Name for no purpose, he should remove his shoes and sit on the ground and humble himself and request from three people that they release him (ibid. in the name of the Ramban). And one who curses using the Name, such as saying "May Hashem punish you" and the like, his punishment is very great (ibid.). And one should not increase blessings in vain, as we have explained in siman 206, section 12, see there (and all the laws that the Magen Avraham wrote here in subsection 6 we have explained there).