In the opinion of Maharam of Rotenberg, while reciting Keriat Shema and its berachot, one may not interrupt to respond to Kaddish and Kedushah, for since he is already engaged in the praise of God, he is prohibited from interrupting to answer another matter of praise. However, according to the majority of the Rishonim (Talmidei Rabbeinu Yonah, Rosh), for the sake of great praises recited in a congregation, a person is permitted to interrupt even in the middle of Birkot Keriat Shema, and that is the halachah (Shulchan Aruch 66:3).
When interrupting, one may only respond to the most important parts of prayer. In Kaddish there are those who say that he may only answer, “Yehei Shemei rabbah mevorach l’alam…” (“May His great Name be blessed forever and ever”). Likewise, he answers Amen after “da’amiran b’alma v’imru Amen” (“that we utter in the world and say Amen”), which is the conclusion of the main part of the Kaddish (Mishnah Berurah 66:17). Others say that he may answer all five Amens that one normally answers in the main part of the Kaddish, which is called Half-Kaddish; however, the ensuing additions are not considered to be the essence of the Kaddish, and he may not interrupt to answer Amen after them (Kaf HaChaim 66:23; and see further in this book 23:14).
In Kedushah, he may only answer by reciting the verses “Kadosh…” and “Baruch…,” the essence of the response to Kedushah (Mishnah Berurah 66:17; Yalkut Yosef 66:2). There are those who say that he may also respond to the verse “Yimloch…” (Aruch HaShulchan 66:6; Kaf HaChaim 18), and that is the widespread custom. However, he must refrain from reciting the introductory words that the chazan says before every verse.
He may respond to Barchu, “Baruch Hashem hamevorach l’olam va’ed.” (“Blessed is Hashem, Who is blessed for all eternity.”) At Modim of the Amidah repetition, he bows down and answers, “Modim anachnu lach” (“We thank you”) and nothing more.
According to the Ashkenazic minhag, one may answer Amen to the berachot, “HaKel HaKadosh” and “Shomeya Tefillah,” which conclude the berachot of praise, and the berachot of request. According to the Sephardic minhag, the law regarding those particular berachot is similar to other berachot, and one does not respond to them.
Concerning this law, there is no difference between being in the middle of a berachah or paragraph, and being between berachot or paragraphs.
All answers are permitted only in the middle of the berachot or after their conclusion. However, once a person says “Baruch Attah Hashem” at the end of the berachah, and there only remain a few words to conclude the berachah, he may not interrupt at all (Bei’ur Halachah 66:3). Similarly, when reciting the verses “Shema Yisrael” and “Baruch Shem kevod…” in which one accepts upon himself the yoke of Heaven, one may not interrupt for anything, for the ruling regarding them is like that of the Amidah, which we do not interrupt at all (Shulchan Aruch 66:1).
In any case of uncertainty, it is best not to answer, for according to many poskim, even if it is permissible to answer, there is no obligation to respond.
. The Torat Chaim (Sofer) 66:8 writes that it is not an obligation to respond, rather one is permitted to do so, as written in Shut Yad Eliyahu and the Maharshag. See Yabia Omer part 1, 5:7 and part 8, 6:1-2. Halichot Shlomo 6:12 writes that even in Pesukei d’Zimrah one is permitted to answer but is not obligated to do so. Additionally, we already learned that according to Maharam of Rotenberg, it is forbidden to answer either Kaddish or Kedushah in the middle of Birkot Keriat Shema, and although the halachah does not follow his opinion, nevertheless, in a situation of uncertainty, one may take his opinion into account and not respond. Concerning the question of whether or not it is preferable for a person to respond, the answer depends. If he is praying in a minyan, and he will hear all the matters of sanctity anyway, then if responding will disturb his concentration, it is best that he does not answer. However, if the people are standing for Kedushah, he must be sure not to appear as though he is separating himself from the congregation, and therefore he must stand with his legs together when they do, and likewise bow at Modim. If another opportunity will not arise to hear Kaddish and Kedushah, it is best that he answers with them (see Mishnah Berurah 66:51). See the laws of Pesukei d’Zimrah in this book, chapter 14:4, and note 7.The Mishnah Berurah 66:23 mentions different opinions regarding whether one is allowed to respond Amen to berachot while in between the passages. Although he tends to agree that one may respond, I have not mentioned this, based on the rule that in a case of uncertainty, “shev v’al ta’aseh adif,” sitting and not performing an action is preferable. The Igrot Moshe, Orach Chaim, part 4, 21:2 rules that one responds Amen to the Kohanim who are reciting the verses of Birkat Kohanim, since the Bei’ur Halachah 128 states that this Amen is a biblical obligation. One who must relieve himself must do so and wash his hands, although he recites Asher Yatzar only after he finishes the Amidah (Mishnah Berurah 66:23). If he is called up to the Torah, according to the Shulchan Aruch 66:4 he does not ascend, although according to the Ashkenazic poskim he does. Even according to the Ashkenazic custom, if he suspects that he will be called up, it is best that he leave the synagogue beforehand (Mishnah Berurah 66:26). If he is in the middle of reciting Pesukei d’Zimrah, the law is more lenient, and if he is called he ascends. If he is the only Kohen or Levi present, he can be called l’chatchilah, as explained in chapter 14:5 of this book.
If a person did not have tallit and tefillin, and they are brought to him after he already began reciting Birkot Keriat Shema, but before he reaches Shema, he must wait until the conclusion of the berachah. According to the Shulchan Aruch, he wraps himself in the tallit and puts on the tefillin with a berachah. According to the Rama, he recites the berachot on the tefillin, but wraps himself in his tallit without reciting the berachah, since there is no obligation to wrap himself in a tallit for Keriat Shema. If the tallit and tefillin were brought to him after he already reached Keriat Shema, there is discussion among the poskim about how to put on tefillin in the middle of a passage so that he will not appear as someone who is giving false testimony even regarding one verse of Keriat Shema. Still, it seems that he can decide in his heart to repeat that same passage from the beginning and then he will be considered as one who is between passages, allowing him to recite the blessing on the tefillin according to all opinions. As an added precaution, he can continue reading until the end of the passage like one who reads the Torah, then put on tefillin with a berachah, and then go back to read with kavanah from the beginning of that same passage. By doing so, he evades the dispute between the Shulchan Aruch and the Rama and he will not lose out on saying the second berachah that is recited according to Minhag Ashkenazim, which he would miss if he were to put on the tefillin in the middle of the passage (see Mishnah Berurah 66:47). If his tallit and tefillin are brought to him in the middle of the Emet V’Yatziv berachah, according to the Shulchan Aruch, he wraps himself in the tallit and he puts on his tefillin without a berachah. According to the Rama, he recites the blessings on his tefillin. The Mishnah Berurah 66:47 writes that it is correct to recite only the “Lehaniach” berachah. If the tallit and tefillin were brought to him between Ga’al Yisrael and the Amidah, he puts on tefillin without a berachah and does not wrap himself in his tallit, so as not to interrupt between redemption and prayer (Shulchan Aruch 66:8).