[The Laws Regarding] In Which Place One May Interrupt and in Which Place One May Not Interrupt. Containing 10 Se-ifim
Between the sections [of Shema or it's blessings], one may greet an honored person and respond to a greeting from any person. And in the middle [of a section], one may greet someone that one reveres, such as one's father, one's teacher, or one greater than oneself in wisdom, and all the more so a king or one who compels. And one may respond to a greeting from an honored person, even in the middle of a verse, except for the verse of "Sh'ma Yisrael" and "Baruch shem kavod malchuto l'olam va-ed", during which one may not interrupt at all unless it is out of fear that he will be killed.
If one forgot to don tzitzit or tefillin, one can interrupt between sections [of Shema or it's blessings] and don them, and one can make a blessing over them. Rem"a: And there are those who say one should not make a blessing over them until after the Prayer [Amidah], and such is the custom with regard to tallit (Beit Yosef)
For Kaddish, K'dushah, and Bar'chu, one interrupts [Shema or it's blessings] even in the middle of a verse. And so too for Modim, but one [should] only say the word "Modim" alone. Rem"a: And also, regarding [answering] Barchu [between or in the middle of sections] (which is allowed), one should not say "yitbarach v'yishtabach, etc." (ie. there is an old custom in some places where the congregation would say "yitbarach v'yishtabach, etc." while the leader is saying Barchu). And there are those who say that the "Amen" that we respond with after the blessing of "ha-El ha-Kadosh" ("the Holy God" - at the end of the K'dushah in Shemona Esrei) and after "shomeya tefilah" ("the one who hears prayer" in Shemona Esrei) has the status of the K'dushah, and one can answer them during the Recitation of the Sh'ma. And that is the main opinion [that should be followed]. And with all these things, we interrupt, and certainly when saying supplications [Teshuvot of the Rashba]
If a Kohen [priest] was reciting the Recitation of the Sh'ma and they called him to the Torah: there is [an opinion] that says that one interrupts [the recitation for the aliyah] and there is [an opinion] that says one does not interrupt. And the halachah is according to his words (ie. the latter).
These are [what are considered] "between the sections": between the first blessing and the second, between the second and Sh'ma, between Sh'ma and V'Haya Im Sh'moa [the second paragraph of the Recitation of the Sh'ma], between V'Haya Im Sh'moa and Va-yomer [the third paragraph]. But between Va-yomer and "Emet V'yatziv", we do not interrupt, because we do not interrupt between YHVH and Emet, rather one should say "ani YHVH Eloheichem emet" and then interrupt as in the case of the middle of a section.
If one stopped because of the feared or honored [person] after they [concluded Shema and] said "emet" or they finished [Shema] before the chazzan, [meaning:] they joined "YHVH Eloheichem" with "emet", and are now waiting for the chazzan to begin [so that they can say it] with him, there is no need to go back and say "emet" another time [because one interrupted the sentence].
One should not answer amen after [the blessing of] "ga'al Yisrael", because this is considered an interruption [between the Recitation of the Sh'ma and the Amidah]. Rem"a: There are those who say that we do respond "amen", and that is our custom, to answer "amen" after the prayer leader. But if one prayed alone, he should not respond amen [to his own blessing], as below in Siman 215 [Tur].
One needs to adjoin [the Blessing of] Redemption to the Amidah, and one may not interrupt after they said "ga'al Yisrael", unless there was an unavoidable circumstance - for example, they did not wear tefillin (ie. they did not have a pair of tefillin up until then) and then [the tefillin] became available between Redemption and the Amidah, [in such a case, they should] don them right then but they should not bless over them until after they pray [the Amidah]. But [regarding] a tallit, one should not put it on at that point. But if one did not yet say "ga-al Yisrael", and now a tallit and tefillin become available, they don them [right then] but should not bless over them until after the Amidah. Rem"a: And there are those who say that prior to "ga-al Yisrael" one should bless over the tefillin , and such is our custom [Mordechi and Tosefot and Hagahot Maimoni from Hilchot Kriat Sh'ma]
One should not respond to Kaddish or Kedushah between Redemption and the Amidah. What should he do? He should wait at [the words] "shira chadasha" in order to respond.
Anyone who did not say "Emet V'yatziv" in the morning service and "Emet V'Emunah" in the evening service has not fulfilled their obligation of the commandment as as it should have been. Rem"a: One who had an unavoidable circumstance and is pressed [for time] and does not have time to pray [Shemona Esrei] immediately after the Recitation of the Sh'ma: they should recite the Sh'ma until [the word] "emet" and then wait to say the rest of the blessing until they can pray [Shemona Esrei]. And then they say "v'yatziv v'nachon" etc. and then pray [Shemona Esrei] in order adjoin Redemption to the [Shemona Esrei] prayer [Beit Yosef in the name of Rokeach, chapter 321].