(1) We [are obligated to] recite the Shema twice daily - in the evening and in the morning - as [Deuteronomy 6:7] states: "...when you lie down and when you rise" - i.e., when people are accustomed to sleep - this being the night - and when people are accustomed to rise, this being daytime.
(2) And what is it that one recites? These three sections:
"Hear O Israel..." (Deuteronomy 6:4-9),
"And if you will listen..." (Deuteronomy 11:13-21),and
"And God said..." (Numbers 15:37-41).
We begin with the section of "Hear O Israel" since it contains [the concept of] the unity of God, [the commandment of] loving Him and the study of Torah, it being a fundamental principle upon which everything is based.
After it, [we read] "And if you will listen...," since it contains the imperative to fulfill the rest of the commandments, and finally the portion of tzitzit, since it also contains the imperative of remembering all the commandments.
(3) The commandment of tzitzit is not obligatory at night. Nevertheless, we recite [the section describing] it at night because it contains mention of the exodus from Egypt.
We are commanded to mention the exodus both during the day and at night as [Deuteronomy 16:3] states: "In order that you shall remember the day of your leaving the land of Egypt all the days of your life."
Reading these three sections in this order constitutes the recitation of the Shema.
(4) When reciting the Shema, after completing the first verse, one says quietly "Blessed be the name of the glory of His Kingdom forever." He then continues to read the first section in its normal fashion: "And you shall love God, your Lord..."
Why do we read it in this fashion? It is our tradition that when the patriarch, Jacob, gathered all his sons together in Egypt close to his death, he commanded and urged them regarding the Unity of God and the path of God upon which Abraham and Isaac, his father, had tread.
He asked them: "My sons, perhaps there are dregs among you, one who does not stand with me in the Unity of God?" This is comparable to the manner in which Moses, our teacher, said to us: "Lest there be among you a man or woman [whose heart turns this day from God...]" (Deuteronomy 29:17).
They all answered and said: "Listen, Israel, God is our Lord, God is One," i.e., listen to us, Israel, our father, God is our Lord, God is One.
The wise elder responded: "Blessed be the Name of the Glory of His Kingdom forever." Therefore, the Jews are accustomed to utter the praise that Israel, the wise elder, uttered after this verse.
(5) Blessings are recited before and after Kri'at Shema. In the day, one recites two blessings before it and one after it. At night, one recites two blessings before and two blessings after it.
(6) The first blessing preceding [the Shema] in the day [begins: "Blessed are You, God...], the One who forms the light and creates darkness,..." The second blessing [begins with]: "With everlasting love, You have loved us..."
[The Shema] is followed by [the section beginning] "True and certain..."
The first blessing preceding [the Shema] at night [begins: "Blessed are You, God...], the One who brings the evening,..." and the second [begins] "With everlasting love, You have loved Your people Israel." The first blessing after [the Shema] is [the section begining] "True and faithful..." and the second [begins] "Lay us down..."
(7) The first blessing preceding [the Shema], both in the day and at night, begins "Blessed [are You, God, our Lord...]" and concludes "Blessed [are You, God]..." The rest of the blessings all conclude with "Blessed [are]...," but do not begin "Blessed [are]..."
These blessings and all the rest of the blessings familiar to the Jewish people were instituted by Ezra, the scribe, and his court. One may not detract from them or add to them.
In every instance that they decreed to conclude with "Blessed...," one may not omit this conclusion. Where they decreed not to conclude [with "Blessed..."], one may not conclude with it. Where they decreed not to begin with "Blessed," one may not begin with it. Where they decreed to begin [with "Blessed..."], one may not omit it.
The general principle is that anyone who deviates from the set form of blessings established by the Sages is mistaken and must recite the blessing again in its proper form.
Anyone who does not say [the paragraph of] "True and certain..." in the morning prayer or [the paragraph of] "True and faithful..." in the evening prayer does not fulfill his obligation.
(8) One who recites the second blessing before the first, whether in the day or at night, or whether the transposed blessings are recited before or after Kri'at Shema, fulfills his obligation, since there is no absolute order to the blessings.
A person who begins with "...the One who forms the light..." and concludes with "...the One who brings the evenings" in the morning prayer does not fulfill his obligation.
Were he to begin with "...the One who brings the evenings" and conclude with "...the One who forms the light", he would fulfill his obligation. Were he to begin with "...the One who brings the evenings" ...and conclude with "...the One who forms the light" in the evening, he would not fulfill his obligtation.
If he begins with "...the One who forms light" and concludes with "...the One who brings the evenings" - he fulfills his obligation since all blessings are defined by their conclusions.
(9) When is the [proper] time for the recitation of Shema at night? The commandment [starts] from the time of the appearance of the stars [and continues] until midnight.
A person who transgresses and delays fulfills his obligation if he recites [the Shema] before dawn. [The Sages established the limit] of midnight only in order to distance us from negligent wrongdoing.
(10) One who reads the Shema [of the night] after dawn, [but] before sunrise, does not fulfill his obligation unless he was unavoidably detained - e.g., drunk or sick, or in a similar situation. A person who was so detained and reads [the Shema] at this time does not recite [the blessing of] "Lay us down."
(11) When is the proper time [for the recitation of the Shema] during the day? The commandment is that one should start to read before sunrise in order to conclude and recite the last blessing with the sunrise. This measure [of time] is one-tenth of an hour before the sun rises.
A person who delays and reads the Shema after the sun rises fulfills his obligation, for the proper time is until the end of the third hour of the day for one who transgresses and delays.
(12) One who is overhasty and recites the Shema of the morning prayers after dawn, even though he finishes before sunrise, fulfills his obligation. In extraordinary circumstances - e.g., one who rises early in order to travel - one may recite it at the outset from dawn.
(13) One who recited [the Shema] after [the end of] the third hour, even if he was unavoidably detained, does not fulfill his obligation to recite the Shema at its proper time. He can be compared to one who studies Torah.
He should recite the blessings preceding it and after it all day, even if he delays and recited it after [the end of] the third hour.