One must stand while reciting Hallel, because Hallel is a testimony to God’s glory, and witnesses must stand while testifying. Be-di’avad, if one recited Hallel sitting or lying down, he has nonetheless fulfilled his obligation. One who is ill and cannot stand may recite it sitting or lying down le-khatĥila (sa 422:7, mb ad loc. 28).
One should not interrupt one’s recitation of Hallel, even just by remaining silent. In the case of an important need, however, like preventing an insult, one may interrupt his recitation. One may also interrupt Hallel in order to recite devarim she-bikdusha (“sacred words”; e.g., Kedusha, Kaddish, Barkhu, or Modim). One should recite Hallel in order, from beginning to end. One who recited it out of order has not fulfilled his obligation and must return to the place where he deviated from the proper order and read it in order (sa 422:4-6). It is proper to read Hallel slowly and pleasantly, and many congregations have a custom to sing parts of it.
The Sages ordained that it is proper that one recite Hallel immediately following the Amida of Shaĥarit. Since we mention the uniqueness of Rosh Ĥodesh in the Amida, in Ya’aleh Ve-yavo, it is appropriate to continue praising God immediately and continue thanking Him for sanctifying Israel and the New Moons. Be-di’avad, one may recite it later in the day, because the entire day is suitable for reciting Hallel (Megilla 20b).
There are various customs regarding how to recite Hallel: which verses are repeated, which verses are recited responsively, etc. All the customs are legitimate and every community should continue following its custom (Sukka 38a-39a, sa 422:3).
Customarily, the ĥazan reads four verses aloud:
Thank the Lord, for He is good, His loving-kindness is forever.
Let Israel say His loving-kindness is forever.
Let the house of Aharon say His loving-kindness is forever.
Let those who fear the Lord say His loving-kindness is forever. (Tehilim 118:1-4)
According to Ashkenazic custom, the congregation responds, “Thank the Lord, for He is good, His loving-kindness is forever” to each of these verses. According to the Sephardic custom, however, the congregation repeats each verse after the ĥazan.
Regarding the repetition of verses, it has become the accepted custom in the last few generations to repeat each verse from Odekha (“I will thank You”) until the end of Hallel (Tehilim 118:21-29). The reason we repeat these verses is to continue the pattern of the beginning of Tehilim 118, in which every idea is repeated. This pattern ends at “I will thank You,” but we continue the pattern in Hallel by repeating the rest of the verses of that chapter as well. Furthermore, according to the Talmud (Pesaĥim 119a), these verses were composed by David, his father Yishai, and his brothers. Thus, because of the importance of these verses, the Sages instructed us to repeat them.
We repeat the verse, “Lord, please, save us. Lord, please, grant us success” (Tehilim 118:25), in a unique manner, reciting the first part twice and then the second part twice.