Related sur Berakhot 2:12

The grooms and all [others] who are involved in [other] Mitzvot (commandments) are exempt from reading [the] Shema and from [the Amidah] prayer and from Tefillin (phylacteries), as it is said, “… when you sit in your house”, (Deuteronomy 6:7) which excludes grooms, “and when you walk on the way”, (Deuteronomy 6:7) which excludes those who are involved in mitzvot.

The grooms and all [others] who are involved in [other] Mitzvot (commandments) are exempt from reading [the] Shema and from [the Amidah] prayer and from Tefillin (phylacteries), as it is said, “… when you sit in your house”, (Deuteronomy 6:7) which excludes grooms, “and when you walk on the way”, (Deuteronomy 6:7) which excludes those who are involved in mitzvot.

One who reads the Megillah out of order does not fill [his obligation], and so too with Hallel and so too with tefillah (i.e., the Amidah), and so too with the recitation of the Shema. [If] the [public] reader of the Megillah errors or skips over one of its verses, he does not go back and read the verse by itself, but he begins from the same verse and continues until the very end [of the Megillah], and so too with Hallel, and so too with tefillah, and so too with the recitation of the Shema. One who enters the synagogue and found that they had [already] read half [of the Megillah], and he finishes with them, he does not go back and read it from the beginning until the place [where they were when he arrived], but rather he starts from the beginning and continues until the very end, and so too with Hallel, and so too with tefillah, and so too with the recitation of the Shema.

One who reads the Shema must have intent in his heart. Rebbi Achai says in the name of Rebbi Yehudah: “If he had intent in his heart in the first paragraph [of Shema], even though he did not have intent in his heart in the last (i.e. second) paragraph [of Shema], he fulfilled his obligation [of saying the Shema].”

One that reads the Shema backwards [i.e. out of order] does not fulfill his obligation [of saying the Shema]. And the same [applies] to Hallel, to prayer [of Shemoneh Esreh], and to the Megillah [of Esther].

One who was reading the Shema and made a mistake, and skipped in it one verse, [he] should not begin to read that verse by itself, but rather should go back to that verse and complete [the Shema, from that point on] until the end. And the same [applies] to Hallel, to prayer [of Shemoneh Esreh], and to the Megillah [of Esther]. One who entered a synagogue and found that [the congregation] has read half of it (i.e. the Shema) and [he] completed [the remaining half of the Shema] with them, [he] should not go back and read it (i.e. the Shema) from the beginning until that place, but rather [he] should begin from the beginning and complete it until the end. And the same [applies] to Hallel, to Tefillah (prayer) [of Shemoneh Esreh], and to the Megillah [of Esther].

One who reads the Shema and makes a mistake, and [he] does not know where he made the mistake, [he] should go back to the beginning of the [first] paragraph. If he made a mistake in the middle of a paragraph, he should go back to the beginning of [that] paragraph. If he made a mistake between the first [verse that mentions] writing and the second [verse that mentions] writing, he should go back to the first [verse that mentions] writing.

A carrier, even though his burden is on his shoulder, he may read [the Shema]. But while he is unloading [the burden] and loading [the burden] he should not read, because he is not paying attention [to what he is saying]. Either way [i.e. whether he is carrying or loading, or unloading], he should not pray [the Shemoneh Esreh] until he [completely] unloads [and can stand in one place without holding his load].

A carrier, even though his burden is on his shoulder, he may read [the Shema]. But while he is unloading [the burden] and loading [the burden] he should not read, because he is not paying attention [to what he is saying]. Either way [i.e. whether he is carrying or loading, or unloading], he should not pray [the Shemoneh Esreh] until he [completely] unloads [and can stand in one place without holding his load].

Workers [must] read [the Shema even while they are sitting] on top of a tree. And they [can] pray [Shmoneh Esreh] on top of an olive tree or on top of a fig tree. But [if they are on top of] all other trees, they [must] go down to pray [Shmoneh Esreh]. The owner of the house [must] go down regardless [what kind of tree he is on top of] and pray [Shmoneh Esreh]. Workers [must] read the Shema and say a Beracha (blessing) before it and after it. [When] they eat their bread they [must] say the Beracha before it and after it. And they [must] pray three times [a day] Shmoneh Esreh, but they do not go in front of the pulpit [to lead the congregation].

Best men and bridesmaids, and all [other] wedding attendees are exempt from [the] prayer [of Shmoneh Esreh] and from [putting on] Tefillin (Phylacteries), all seven days [of the wedding celebrations], but [they are] obligated in reading the Shema. Rebbi Shila says, “The groom is exempt [from saying the Shema, Shemoneh Esreh and putting on Tefillin], but the wedding attendees are obligated [in all of them].”

Best men and bridesmaids, and all [other] wedding attendees are exempt from [the] prayer [of Shmoneh Esreh] and from [putting on] Tefillin (Phylacteries), all seven days [of the wedding celebrations], but [they are] obligated in reading the Shema. Rebbi Shila says, “The groom is exempt [from saying the Shema, Shemoneh Esreh and putting on Tefillin], but the wedding attendees are obligated [in all of them].”

“A man who had a seminal emission (Baal Keri) who does not have water to dip in may read the Shema, but he may not [read it loud enough so that he can] hear [himself talking] with his own ear, and does not say the Beracha (blessing) not before it and not after it.” [These are] the words of Rebbi Meir. And the Chachamim (Sages) say, ”He may read the Shema and he may [read it loud enough so that he can] hear [himself talking] with his own ear, and he says the Beracha [both] before it and after it.” Rebbi Meir said, “One time we were sitting in the Bet Midrash (Study Hall) in front of Rebbi Akiva and we were reading the Shema, but we were not saying it loud enough to be able to hear ourselves, because of one inquisitor who was standing by the door.” They (i.e. Chachamim) said [back] to him, “The time of danger is not a proof.”