Berachos, Chapter 2, Mishnah 2. In the previous mishnah we learned about the pauses that are permitted and those that are forbidden, both between the sections and within the sections themselves of Shema and the berachos that surround it. In the mishnah before us the Tanna spells out where exactly those pausing points are, and in addition the mishnah takes up the question of why the order of the passages of Shema was fixed as we have it.
The pausing points - "Eilu hein bein haperakim":
"Bein berachah rishonah lishniyah" - between the first berachah of Shema, which concludes with "yotzer hame'oros," and the second berachah, "Ahavah Rabbah" ("Ahavas Olam").
"Bein shniyah li'Shema" - between the second berachah and the recitation of Shema itself.
"Bein Shema li'Vehayah im shamo'a" - between the first passage and the second passage.
"Bein Vehayah im shamo'a li'Vayomer" - between the second passage and the passage of tzitzis.
"Bein Vayomer le'Emes veyatziv" - between the end of the passage of tzitzis and the berachah of "Emes veyatziv."
And the Tanna adds: "Bein Vayomer le'Emes veyatziv lo yafsik" - one may not pause between "Vayomer" and "Emes veyatziv." For this reason the practice is to say "Hashem Elokeichem emes," joining the word "emes" to the end of the passage without any pause between them.
The reason for the order of the passages:
Rabbi Yehoshua ben Korchah addresses the question of why the order of the passages of Shema was fixed specifically in this way:
"Lamah kadmah Shema li'Vehayah im shamo'a? Ela kedei sheyekabel ol malchus Shamayim techilah, ve'achar kach yekabel ol mitzvos" - first a person must accept upon himself the yoke of the kingship of Heaven, that is, the recognition that the Holy One runs the world and runs his own life, and that he is therefore obligated to listen to His word. Once this recognition is firmly in place, he is able to accept upon himself the yoke of the mitzvos and the commandments.
"Vehayah im shamo'a" precedes "Vayomer" - "she'Vehayah im shamo'a noheg bayom uvalailah, ve'Vayomer eino noheg ela bayom" - the passage of "Vehayah im shamo'a" deals with mitzvos that apply both by day and by night, such as Torah study, while "Vayomer" deals with the mitzvah of tzitzis, which is a mitzvah that depends on the daytime. It was therefore placed after "Vehayah im shamo'a."
In summary: This mishnah listed the pausing points "between the sections" - between the first berachah and the second, between the second berachah and Shema, and between one passage and the next; and we learned that one may not pause between "Vayomer" and "Emes veyatziv," which is why "Hashem Elokeichem emes" is joined together. We also examined the reason for the order of the passages: accepting the yoke of the kingship of Heaven comes before accepting the yoke of the mitzvos, and a passage that applies both by day and by night comes before a passage that applies by day alone.