Care must be taken to recite Kriat Shema during the proper time (between 4:33 - 8:22am (Magen Avraham) or 4:33 – 9:06am (Gra & Baal HaTanya). One who goes to sleep prior to davening should ensure that he awakens in time to recite Shema.
Full Hallel is recited on Shavuot. When reciting Hallel, one may not interrupt it – other than for those things that one may respond to during the Brachot of Kriat Shema. [I.e. If the Chazzan recites Kedushah, one responds Kadosh, Baruch and Yimloch. When the Chazzan says Ha’E-l Hakadosh, one answers Amen. When the Chazzan says Modim, one answers the three words Modim Anachnu Lach. When the Chazzan recites Kaddish, one answers Amen Yehei Shmei etc, and Amen to d’amiran b’alma. One also answers Barchu and Amen when the Brachot are recited before and after an Aliyah or Haftorah.]
Hallel is recited standing (unless one is feeble or infirm).
It is preferable to recite Hallel with the Minyan. If one is not up to the Minyan, this creates a dilemma; on the one hand it is appropriate to recite Hallel with the congregation (if he hasn’t yet begun Baruch She’amar), and on the other hand, it is appropriate that one davens in the correct order. For this reason, the Chabad Rabbeim were punctilious about being up to Hallel when the Minyan recited it on Chag.
If one forgot to recite Hallel, he should recite it with a Brachah as soon as he remembers, as long as it is not yet sunset 7:49pm (or without a Brachah until Tzeit).
If one mistakenly recited half-Hallel, he must repeat the entire Hallel (without a Brachah).
