The Rishonim write that before Barchu it is customary to recite three verses, which open with the words, “V’Hu Rachum yechaper avon,” (“He, the merciful One, atones iniquity”), to request atonement for the sins that we committed throughout the day. Furthermore, at night, the Divine attribute of judgment is in force and mechablim (destructive spiritual agents) are given permission to harm. Therefore, we ask, “V’Hu Rachum yechaper avon…” (see Tur and Beit Yosef 237 and Kaf HaChaim 235:5). On Shabbat and Festivals, V’Hu Rachum is not recited.
According to the Sephardic minhag, three verses beginning with the words, “Hashem Tzevakot…” are recited before the beginning of Ma’ariv, and according to the minhag of the Chassidim, “Shir HaMa’alot…” is recited. Followers of both minhagim recite Half-Kaddish afterwards and then V’Hu Rachum. If Torah learning was conducted before the prayer service, at the end of which Kaddish Al Yisrael was recited, it is unnecessary to recite Half-Kaddish as well, so as to avoid saying too many Kaddishim (Yalkut Yosef, part 3, 236:1).
By reciting Barchu we introduce Birkot Keriat Shema, and therefore it is forbidden to talk after Barchu, similar to the law concerning someone who is in the middle of a passage of the Shema (Mishnah Berurah 236:1; 54:14; see earlier in this book 16:4). Therefore, whoever did not succeed in saying V’Hu Rachum before Barchu does not recite it after Barchu, so as not to interrupt in the middle of Birkot Keriat Shema (see Yabia Omer 2:5).
The majority of Sephardim have the custom not to answer Amen after the berachot of the chazan to prevent interruption in the middle of Birkot Keriat Shema. It is best to finish the berachah with the chazan or slightly after him, so that according to all opinions there will be no need to respond Amen. After Birkat Hashkiveinu, some answer Amen (Yalkut Yosef, part 3, 236:6) and some do not (Ben Ish Chai, Pekudei 5). According to the Ashkenazic minhag, Amen is answered after the chazan’s blessings, and is not considered an interruption. However, even the Ashkenazim try not to answer Amen after Birkat Ahavat Olam so as not interrupt between the berachah and Keriat Shema, and they do this by finishing the berachah together with the chazan or after him (see earlier in this book 16:4; all matters concerning Birkot Keriat Shema are clarified in chapter 16).
At the end of Birkat Hashkiveinu, Sephardim are accustomed to responding Amen to their own blessing, since it is a conclusion of a series of berachot. However, Ashkenazim do not answer Amen to their own berachot, with one exception, following Birkat Boneh Yerushalayim in Birkat Hamazon (Shulchan Aruch 215:1; 236:4).
Between Birkot Keriat Shema and the Amidah, the chazan recites Half-Kaddish, and after the Amidah he says Kaddish-Titkabal. After that, according to the Sephardic minhag, Shir HaMa’alot Esa Einai is recited, following which those in mourning recite Mourner’s Kaddish, one of whom says Barchu. Subsequently, Aleinu L’Shabe’ach is recited and there is no Kaddish said after it. According to the Ashkenazic minhag, immediately after Kaddish-Titkabal, Aleinu L’Shabe’ach is recited, followed by the recital of the Mourner’s Kaddish by the mourners, one of whom says Barchu.