For me, these two paragraphs - paragraphs 19 and 20 of Section 167 - were the most difficult to translate of all the paragraphs we have encountered in this study group so far. It is not that their content is recondite or particularly convoluted: it is because the word order is so very different from an acceptable word order in modern English. For this reason, in the translation, I have taken small liberties here and there so that the intention of these paragraphs will be clear to the modern English-speaking reader.
On several occasions during this series of shiurim and in connection with several topics we have had occasion to refer to the halakhic possibility that one person can "enable" another person to fulfill a mitzvah. But this is the case not only with regards to a mitzvah but, as we have seen, also with regards to a berakah. [For examples see Shabbat 025, Shabbat 026 and Shabbat 047.] However, in order to understand the content of paragraphs 19 and 20 of Section 167, we must refine somewhat our understanding of the possiblility of "enablement". The conceptual basis for the possibility that one Jew can fulfill a mitzvah through the agency of another Jew is to be found in the concept enunciated in the Gemara [Sanhedrin 27b] that "all Jews are responsible for each other". The idea is that when our ancestors received the Torah at Sinai they undertook a general and collective responsibility for Torah observance. Since there is a "responsibility" devolving upon me to ensure that another Jew be able to observe a mitzvah for which we are mutually obligated, there is no reason why I should not be able to perform the duty as his agent, as it were, at his request or with his consent. However, this is based upon the concept of equal duty. Where there is no duty, no mitzvah, I cannot "enable" my fellow Jew.
There are several categories of berakhah. Some benedictions are associated with the performance of a religious duty; other benedictions are associated with sheer praise of God. But there is a third category which is relevant to our present discussion. The sages set down a rule that one may not enjoy any of the good things of this world without first reciting a berakhah of recognition and gratitude. Since enjoyment of the good things of this world (fruit, vegetables and so forth) is a matter of personal choice the concept of mutual responsibility cannot apply. From day to day and from week to week I am not required to eat meat or fish (indeed, personally I do not eat meat or fish!) or fruit or vegetables or confectionary - or anything else: this is my personal choice and preference. Therefore only I can recite the associated berakhah - whatever it may be.
We can now apply these concepts to the contents of paragraphs 19 and 20 above. The blessing before eating bread is one of those birkhot ha-nehenin - blessings recited before enjoying the good things of this world. Therefore, if I am not going to eat bread I cannot recite the berakhah on behalf of someone else. (Of course, if I am going to eat bread I can recite the berakhah out loud and anyone else present can respond Amen and this would be considered as if he had also recited that berakhah. We have seen this on several occasions already.) This whole issue is discussed very succinctly in the Gemara [Rosh ha-Shanah 29b] and there it is pointed out that reciting one of the birkhot ha-nehenin together with children is permitted (even if one is not required to recite that benediction): we all have an obligation to educate children in Jewishness.
However, there are occasions when halakhah requires us to eat certain foods. Here I am not referring to various customs and traditions - such as the eating of suggestive foods on Rosh ha-Shanah or the eating of doughnuts on Ĥanukah or the eating of hamantaschen on Purim. These are pure folk custom (and some may be "more honoured in the breach than in the observance". I am referring to occasions when halakhah - indeed Torah in the strictest connotation of the term - commands us to eat a certain food. The most obvious example is the requirement of the Torah [Exodus 12:15 etc] that we eat matzah at the Seder service. While the actual duty of eating matzah cannot be delegated to someone else (because it is a mitzvah that we must perform with our own bodies), the recitation of the accompanying berakhah can be recited by someone else on my behalf even if they themselves are not at this moment fulfilling that mitzvah. (Possibly, they have already done so.)
Now it may be objected that Rabbi Karo himself, in paragraph 20, admits that the eating of ĥallah on Shabbat and YomTov is a mitzvah: therefore surely it should be possible for someone who is not eating ĥallah herself to recite the berakhah out loud to enable the others to respond Amen. However, this is not the case. The mitzvah is not specifically to eat bread - or anything else - on Shabbat. The mitzvah is the general one of oneg Shabbat, of making Shabbat a delightful day [Isaiah 58:13]. The most obvious way of making Shabbat a delightful day is to have three satisfying meals [shalosh se'udot] and eating them in a celebratory manner - which would include the eating of bread before the meal and the reciting of grace after the meal. But someone for whom eating thus would be the opposite of making Shabbat a delight is entitled to refrain. Thus the eating of ĥallah before the meals on Shabbat is "required" only as part of the general oneg Shabbat. Therefore, someone who is not themselves eating the ĥallah cannot recite the benediction on behalf of the others.
Rabbi Moshe Isserles adds a note to the end of paragraph 20 which returns the paragraph to the topic of Shabbat, after the brief excursus into Passover. The slice of bread over which the benediction has been recited should be the slice of bread that one eats immediately afterwards. He gives the reasons. We should perhaps note here a kabbalistic custom which originates with Rabbi Yeshayah Horowitz [1565-1630]. In his book "Shenei Luĥot ha-berit" he suggests that just as at the Seder service we end the meal with the taste of matzah in our mouths (the afikoman) so, on Shabbat, we should end the meal with the taste of ĥallah in our mouths. I do not know how widespread this custom is today.