(1) It is a positive mitzvah from the Torah to bless [God] after eating satisfying food, as [Deuteronomy 8:10] states: "When you have eaten and are satiated, you shall bless God, your Lord."
The Torah itself requires a person to recite grace only when he eats to the point of satiation, as implied by the above verse, "When you have eaten and are satiated, you shall bless...." The Sages, however, ordained that one should recite grace after eating [an amount of bread equal] to the size of an olive.
(2) Similarly, the Rabbis ordained that we recite blessings before partaking of any food. Even when one wants to eat the slightest amount of food or drink, one should recite a blessing, and then derive benefit from it.
Similarly, when smelling a pleasant fragrance, one should recite a blessing and then smell. Anyone who derives benefit [from this world] without reciting a blessing is considered as if he misappropriated a sacred article.
The Rabbis also ordained that one should recite a blessing after eating or drinking, provided one drinks a revi'it and eats a k'zayit. A person who [merely] tastes food is not required to recite a blessing before partaking of it or afterwards unless he partakes of a revi'it.
(3) Just as we recite blessings for benefit which we derive from the world, we should also recite blessings for each mitzvah before we fulfill it.
Similarly, the Sages instituted many blessings as expressions of praise and thanks to God and as a means of petition, so that we will always remember the Creator, even though we have not received any benefit or performed a mitzvah.
(4) Thus, all the blessings can be divided into three categories:
a) blessings over benefit;
b) blessings over mitzvot;
c) blessings recited as expressions of praise and thanks to God and as a means of petition, so that we will always remember the Creator and fear Him.
(5) The text of all the blessings was ordained by Ezra and his court. It is not fit to alter it, to add to it, or to detract from it. Whoever alters the text of a blessing from that ordained by the Sages is making an error.
A blessing that does not include the mention of God's name and His sovereignty [over the world] is not considered a blessing unless it is recited in proximity to a blessing [which meets these criteria].
(6) All the blessings may be recited in any language, provided one recites [a translation of] the text ordained by the Sages. [A person who] changes that text fulfills his obligation nonetheless - since he mentioned God's name, His sovereignty, and the subject of the blessing - although he did so in a ordinary language.
(7) A person should recite all the blessings loud enough for him to hear what he is saying. Nevertheless, a person who does not recite a blessing out loud fulfills his obligation, whether he verbalizes the blessing or merely recites it in his heart.
(8) Whenever one recites a blessing, one should not make an interruption between the blessing and the subject for which the blessing is recited. If one makes an interruption with other matters, one must recite the blessing again.
If, however, one makes an interruption which relates to the subject of the blessing, one does not have to repeat the blessing. What is implied? When a person recites a blessing over bread and before eating says, "Bring salt," "Bring food," "Give so-and-so to eat," "Bring food for the animal," or the like, he need not repeat the blessing.
(9) A person who is ritually impure is permitted to recite all the blessings. This applies regardless of whether the impurity is of a type from which one can purify oneself on the same day or not.
A person who is naked should not recite a blessing until he covers his genitals. To whom does this apply? To men. Women may recite blessings [while naked], provided they sit with their genitals facing the ground.
(10) [The following principle applies to] all blessings: Although a person has already recited them and fulfilled his own obligation, he may recite them again for others who have not fulfilled their obligation, so that they can fulfill their obligation.
There is, however, one exception: blessings over benefit which is not associated with a mitzvah. In this instance, one may not recite a blessing for others unless one enjoys benefit together with them. Nevertheless, one may recite blessings for benefit which is associated with a mitzvah - e.g., eating matzah on Pesach and reciting kiddush [on Sabbaths and festivals] - for others. They may then eat or drink, even though the one [who recites the blessing] does not eat or drink with them.
(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.