Kitzur Shulchan Aruch, Rabbi Shlomo Ganzfried
Siman 16: Laws Concerning Interruptions During the Berachos of Kerias Shema and During Kerias Shema
Kerias Shema and its three berachos which are: Yotzer or, Ahava rabha, [and] Ernes veyatziv (likewise at the rna'ariv service—Kerias Shema and its berachos) are more stringent [in regard to interruptions] than pesukei d'zimrah. I They (i. e. Kerias Shema and its herachos ) are divided into "sections." The following are considered "between sections." between Yotzer hameoros and Ahava rabba2 between habocher beamo Yisrael beahava3 and Shema Yisrael, between uvish'arecha and ve'haya im shaino'a,4 [and] between al ha'aretz and vayomer.
"Between sections" you are permitted to answer Amein to any berachah that you hear. And, of course, you are permitted to respond to Kedushah, to Kaddish and to Barechu. But, Baruch hu uvaruch shemo may not be said.7 Also, if you hear the congregation reciting Kerias Shona, you should not say the [entire] verse of Shema Yisrael with them, but, rather say, whatever you are reciting, aloud, in the same manner that the congregation is saying Shema Yisrael, so that it appears as if you are reading along with them.
In "mid-section" you should not respond Amein except following the berachah—hael hakadosh, and following the berachah Shomea Tefillah. [Upon hearing] Kaddish you may respond: Yehei shemei rabha mevorach le'olam ule'olinei almaya; and when the chazzan says: da'arniran he'alma ve'imru amein, you may also respond: Amein. However, to the rest of the Amein's of the Kaddish, you may not respond, because they are not of the essence of the Kaddish. [Upon hearing] Kedushah, you should remain silent and listen to the chazzan, and [then] say with the congregation: Kadosh, kadosh, kadosh Hashem Zeva'os mei() chol haaretz kevodo and then remain silent; [then] say along with the congregation: Baruch k'ved Hashem mimekomo. you may not say the rest9 for it is not of the essence of Kedushah. Upon hearing Barechu from the chazzan, or from one who was called to [the reading] of the Torah, you may respond: Baruch Hashem harnevorach le'olam va'ed, as well as Amein which follows the berachos of the oleh. 10 If the congregation is saying Modim, you should also bow [with them] and say: Modim anachnu lach, but no more." Upon hearing the sound of thunder, some hold, that you are permitted to interrupt and recite the berachah.12 Others maintain that you may not interrupt for this.
If these interruptions which are permitted in mid-section, if they occur in the berachos [of Shema], you should manage to make the interruption at the completion of a subject; and if it is during Kerias Shema, you should manage to make the interruption between verses. If this is not possible, [then] you may interrupt even in mid-sentence, and then start again from the beginning of the verse.
The rule we stated [above] that it is permitted to interrupt in mid-section for those things which we enumerated above does not apply to the verse: Shema Yisrael etc. and: Baruch sheim keyed malchuso le'olam va'ed, because when reciting these two verses it is prohibited to interrupt in any circumstance. Even if the king greets you, you may not interrupt to respond. Between Ani Hashem Elokeichem and Ernes veyatziv you should also not interrupt because Scripture states: 13 "And Adono-y Elohi-m is true." Therefore, [to fulfill this verse], it is not permitted to interrupt between elokeichem and emes. It is preferable to take care not to interrupt until after you have also said the word veyatziv (because it also means "truth.") Afterwards after having said emes veyatziv you may interrupt just as any other place in mid-section. (The practice of greeting or responding to a greeting nowadays is not insisted upon and you should not interrupt for this purpose even [when you are] between sections.)