(11) Whenever a person listens to the entire recitation of a blessing with the intention of fulfilling his obligation, he is considered to have fulfilled his obligation although he does not answer Amen. Whoever answers Amen to a blessing recited by another person is considered as if he recited the blessing himself, provided the person who recites the blessing is obligated to recite that blessing.
If the person who recites the blessing is obligated only because of a Rabbinic ordinance, while the person responding is obligated by Torah law, the listener cannot fulfill his obligation until he repeats in response [to the one reciting the blessings] or until he hears [the blessing recited] by someone who, like him, is obligated by Torah law.
(12) When many people gather together to eat [a meal with] bread or to drink wine, and one recites the blessing while the others respond Amen, they are [all] permitted to eat and drink. If, however, they did not intend to eat together, but rather they each came on their own initiative, although they all eat from a single loaf of bread, each one should recite the blessings [before eating] by himself.
When does the above apply? With regard to bread and wine. With regard to other foods, however, which do not require [premeditated intent] to be eaten together as a group, if one person recited a blessing and everyone answered Amen, they may eat and drink although they did not intend to gather together as a group.
(13) Whenever a person hears a Jew recite a blessing, he is obligated to respond Amen, although
a) he did not hear the blessing in its entirety,
b) he was not obligated to recite that blessing himself.
One should not respond Amen if the person reciting the blessing is a gentile, an apostate, a Samaritan, a child in the midst of study, or an adult who altered the text of the blessing.
(14) Whenever responding Amen, one should not recite a rushed Amen, a cut off Amen, nor a short or a prolonged Amen, but rather an Amen of intermediate length.
One should not raise one's voice above that of the person reciting the blessing. Whoever did not hear a blessing that he is obligated to recite should not answer Amen together with the others.
(15) Whoever recites a blessing for which he is not obligated is considered as if he took God's name in vain. He is considered as one who took a false oath, and it is forbidden to answer Amen after his blessing.
We may teach children the blessings using the full text. Even though in this manner, they recite blessings in vain in the midst of their study, it is permissible. One should not recite Amen after their blessings. A person who answers Amen after their blessings does not fulfill his obligation.
(16) It is demeaning for a person to recite Amen after his own blessings. When, however, one concludes the last of a series of blessings, it is praiseworthy to answer Amen - e.g., after the blessing, Boneh Yerushalayim in grace, and after the final blessing [following] the recitation of the Shema in the evening service. Similarly, always, at the conclusion of the last of a series of blessings, one should recite Amen after one's own blessing.
(17) Why is Amen recited after the blessing Boneh Yerushalayim, although it is followed by the blessing Hatov v'hametiv? Because the latter blessing was ordained in the era of the Mishnah and is considered to be an addition. The conclusion of the essential blessings of grace is Boneh Yerushalayim.
Why is Amen not recited after the blessing Ahavat olam? Because it is the conclusion of the blessings recited before the Shema. Similarly, in other instances when [a series of] blessings are recited before a practice - e.g., the blessings recited before the reading of the Megillah or the kindling of the Chanukah lights - Amen [is not recited] lest it constitute an interruption between the blessings and [the fulfillment of] the performance over which they are being recited.
(18) Why is Amen not recited after the blessing over fruits and the like? Because it is only a single blessing, and Amen is recited only after a concluding blessing that follows another blessing or blessings - e.g., the blessings of the king or the blessings of the High Priest - to signify the conclusion of the blessings. Therefore, reciting Amen is appropriate.
(19) When a person eats a forbidden food - whether consciously or inadvertently - he should not recite a blessing beforehand or afterward.
What is implied? If one eats tevel - even food that is classified as tevel by Rabbinical decree, the first tithe from which terumah was not separated, or the second tithe or sanctified foods that were not redeemed in the proper manner, one should not recite a blessing. Needless to say, this applies if one ate meat from an animal that was not ritually slaughtered or was trefah or if one drank wine used as a libation for idol worship.
(20) If, however, a person ate d'mai, although it is fit only for the poor, the first tithe from which terumat ma'aser was separated, even though the proper amount for terumah was not separated because the tithe was taken while the grain was still in sheaves, or the second tithe or sanctified food that was redeemed, but an additional fifth was not added upon it, one should recite a blessing beforehand and afterwards. The same applies in other similar situations.