Berachos, Chapter 2, Mishnah 1. The Mishnah opens with the case of someone who was reading in the Torah, and while he was reading the time for Shema arrived, and he found himself holding in the portion of Shema: "im kivein libo - yatza; ve'im lo - lo yatza" - if he directed his heart, he has fulfilled his obligation; and if not, he has not. That is, if he directed his heart, he has fulfilled the mitzvah of Krias Shema, and if not, he has not.
Explaining "im kivein libo":
What does the Mishnah mean by this? It depends on the Gemara's dispute over whether "mitzvos require intent" or "mitzvos do not require intent" - in other words, whether fulfilling a mitzvah requires an intention to fulfill it, or whether the act of the mitzvah itself is enough:
According to the view that "mitzvos require intent": the Mishnah reads simply. This person was reading in the Torah, and up to that point his intention was not to fulfill the mitzvah of Krias Shema. Therefore, if upon reaching the portion of Shema he directed his heart to fulfill the obligation of the mitzvah, he has fulfilled it, and if not, he has not.
According to the view that "mitzvos do not require intent": even without intent to fulfill the mitzvah, the act itself fulfills the mitzvah, so "if not, he has not fulfilled it" cannot be explained as referring to a lack of intent for the mitzvah. Therefore, according to this understanding the meaning is that he must intend to read the passages properly, in contrast to a person who reads them in order to check whether any words are missing, who is liable to pronounce the words incorrectly in the course of verifying that they were written correctly. It emerges that he must read the passage properly in order to fulfill his obligation.
Interruptions during Krias Shema:
The Mishnah continues and discusses which interruptions are permitted or forbidden in the middle of Krias Shema. It teaches that both in Krias Shema itself and in the berachos surrounding it there are transitions between the sections, and the law governing these transitions differs from the law governing someone standing in the middle of a section:
The view of Rabbi Meir: one who is between the sections may initiate and greet someone whose honor he is obligated in - his father, his mother or his teacher, and he certainly may respond to them if they greeted him. But one who is in the middle of a section may initiate a greeting only toward someone whom he has reason to fear (a government official and the like), and he may respond.
The view of Rabbi Yehudah: Rabbi Yehudah is more lenient. In the middle of a section, he greets out of fear, as Rabbi Meir said, but he responds even out of honor. And between the sections, he greets out of honor, as Rabbi Meir said, and he returns a greeting to any person. The reason: responding carries a greater degree of obligation, since once the other person has opened with a greeting it falls upon the one reading to answer him, and therefore he may respond to any person.
Two explanations of the terms honor and fear:
As explained above: honor refers to his father or his teacher, and fear refers to an official figure.
And some understand it the other way around: fear refers to his father and mother, whom he is commanded to fear, and honor refers to a person who is distinguished in the eyes of others generally, and this is not someone in whose honor he is obligated in his own right.
In summary: in this Mishnah we learned the law of one who is reading in the Torah when the time for Krias Shema arrives, and the two ways of explaining "im kivein libo" - according to the dispute over whether mitzvos require intent, or that the intent refers to reading the passages properly. We also examined the laws of interrupting Krias Shema: Rabbi Meir's distinction between between the sections (greeting out of honor) and the middle of a section (greeting out of fear), Rabbi Yehudah's leniency regarding responding, and the two explanations of the terms honor and fear.