The Nussach of Hallel:
The language: From the letter of the law, Hallel may be recited in any language. One fulfills his obligation even if he does not understand the language that he is reading. [Nevertheless, it is a Mitzvah Min Hamuvchar to recite Hallel in Lashon Hakodesh.]
Which verses in Hallel are to be repeated? There are different customs regarding which verses are said twice. Each community is to follow their custom. [The widespread custom is to repeat each verse from Odecha until the conclusion of Hallel, by the verse of Hodu Lahashem. This is likewise the Chabad custom.]
Which parts are to be read aloud by the Chazan and repeated by the congregation? There are different customs regarding which verses are said aloud by the Chazan and then repeated by the congregation. Each community is to follow their custom. [The Chabad custom is for the Chazan to recite Hodu and for the listeners to recite Hodu and Yomar Nah. The Chazan repeats the verse of Hodu together with the congregation. The Chazan then recites Yomar and the listeners answer Hodu and Yomru Nah Beis. The Chazan then recites Yomru Nah Beis and the listeners answer Hodu and Yomru Nah Yirei. The Chazan then recites Yomru Nah Yirei and the listeners answer Hodu. The four phrases beginning Ana Hashem are each recited by the Chazan and repeated by the congregation. The stanzas are to be recited clearly by the Chazan and then answered by the congregation. Hence, the singing that customarily takes place during this recital should not drown out the voice of the Chazan or the congregation.]
One who is praying without a Minyan: The verse beginning Hodu LaHashem is repeated after each of the next three verses, even when one is praying in private.
Saying Hodu with two other people when praying privately: If one is reciting Hallel without a Minyan it is a Mitzvah to have another two people listen and answer [for Hodu and Ana]. This applies on all days that Hallel is recited, whether the complete or incomplete Hallel. [If however one cannot find another two people easily, then he is not required to trouble himself any further. Even one’s wife, and children that have reached Chinuch, join to make up the other two people needed for this purpose. However, it is a Mitzvah Min Hamuvchar to say it with two other men that are above the age of Mitzvos. Even if one has already recited Hallel he can join to answer for Hallel.]
The word Al in the concluding paragraph: In the concluding paragraph the word “al” should be omitted.
Saying “Avraham Zakein” after Hallel: See Halacha 7A!
 Admur 62:2; Tosafus Sotah 32a; Kneses Hagedola 422:7; Peri Chadash; Kaf Hachaim 422:26
 Bach 193
 Michaber 422:3; Mishneh Sukkah 38a
 The reason for repeating verses: Many of the verses of Tehillim that are recited in Hallel from Hodu Lahashem and Min Hameitzar are repeated within the Psalm by David Hamelech. Such as the words “Tov lachasos Bahashem.. Tov Lachasos, and other verses of the like. Therefore some places are accustomed to also repeat the latter verses starting from Odecha. [Rashi 38a]
 Tur 422; M”B 422:19; Shaar Hakavanos p. 103; Kaf Hachaim 422:43
Other customs: In Sefarad the custom is not to repeat the verses of Baruch Haba and Keil Hashem. [Tur ibid; See Kaf Hachaim ibid]
 Michaber 422:3
The recital of verses after the Chazan is recorded in the Gemara Sukkah 39b. The reason behind this repetition is to serve as a reminder of the original Takana of the Sages in the recital of Hallel. Originally the congregation would simply listen to the Hallel of the Chazan and recite the word Hallelukah after every stanza. There were a total of 123 Hallelukas said by the congregation. [Rambam Chanukah 3:12] In addition, they would repeat the first verse of each paragraph after the Chazan [Roshei Perakim]. In remembrance of this custom it is accustomed even today to repeat the verse of Hodu after the Chazan. [See Sukkah 39b; Rashi “Mikan”]
 The custom used to be for only the Chazan to recite the stanzas of Yomar, Yomru Nah Beis and Yomru Nah Yirei. The congregation would listen and hence fulfill their obligation. Nevertheless, since at times the congregation does not listen properly to the Chazan’s recital, as well as that the Chazan at times does not have in mind to fulfill their obligation, therefore, it is better for each person to say the Stanzas to himself. [M”A 422:8; M”B 422:20; See Kaf Hachaim 422:44] Practically, so is the custom today.
 Hayom Yom 30th Tishreiy; Sefer Haminhagim p. 81 [English]; M”B 422:20
 Sefer Haminhagim p. 81 [English] p. 41 [Hebrew]; Shevach Hamoadim p. 226-227; Otzer Minhagei Cahabad p. 198; Siddur Tehilas Hashem Hachadash; Omitted from Rebbe’s Hagadah and Hayom Yom ibid; See Hearos Ubiurim Ohalei Torah 791:64; 819:73 [that Rav Groner states the custom is for the Chazan not to repeat it]; 1088
 See Kaf Hachaim 422:45 in name of Tosafus 38b and Kol Bo regarding why it is permitted to stop in middle of the verse
 See Hiskashrus 439; Siddur Rav Raskin p. 478
 Sefer Haminhagim p. 69 [English]; Hayom Yom 30th Tishreiy-1st of Cheshvan
 Rama 422:2; Rama 479:1 regarding the night of the Seder; Admur 479:6 regarding the night of the Seder; See also Sefer Haminhagim p. 41
 Admur ibid; M”B 422:17
Other opinions: The custom is to recite the stanzas of Hodu but not of Ana. [Rama ibid; Gr”a; M”B 422:18; Kaf Hachaim 422:41
 The reason: On days that one does not recite the complete Hallel this is to be done as there are opinions that say that a Zimun of three is considered a congregation which warrants a blessing to be said, and hence in order to say a blessing according to all one should strive to have at least three people. [Lechem Chamudos brought in M”A 422:7; M”B 422:18; Kaf Hachaim 422:39] Furthermore, it is to be done even on days that we recite the complete Hallel in order so one is able to read the Hodu to another person. [Igur 815; Shivlei Haleket brought in M”A ibid] Alternatively, it is because it is a Mitzvah to answer Roshei Perakim when reciting Hallel. [Mordechai brought in M”A ibid]
 M”A 422:7; Admur ibid regarding the night of the Seder
Other opinions: Some Poskim rule the above law only applies to days when the incomplete Hallel is recited due to the first reason cited above. [Lechem Chamudos ibid]
 Admur ibid; Elya Raba 422:11; Kaf Hachaim 422:39
 Admur ibid; M”A 479:2; Taz 479:3; Igur 815; Rosh 10:32
Does this apply to all Hallel or only to Hallel of the night of the Seder? The M”A ibid implies that only the night of the Seder this applies, being that women are obligated in the Mitzvah of Hallel on the night of the Seder just like men. However, from the Iggur ibid it is implied that this law applies by all Hallel and not just the night of the Seder. So is also implied from the Taz ibid who states that only the men are obligated in the Hallel, unlike the M”A ibid. Vetzrauch Iyun!
 Admur ibid; M”A ibid; Taz ibid; Igur 815
 Admur ibid
 Sefer Haminhagim p. 69 [English]; Hayom Yom 1st Teves based on gloss of Tzemech Tzedek; Shaar Hakolel 37:8; Siddur Rav Anram Gaon; Rasag; Rambam Megillah 3:10; Abudarham; Rebbe in Haggada Shel Pesach p. 46; See Igros Kodesh 2:147 that Rav Yaakov Landa claimed the Rebbe Rashab would say the word Al, although the Rebbe Rayatz did not do so.