The recitation of Shema includes three paragraphs. The first is “Shema” (“Listen”; Devarim 6:4-9), in which we accept the yoke of heaven and which discusses God’s unity and our love for Him. The second is “Ve-haya im Shamo’a” (“And if you follow”; Devarim 11:13-21), which contains the acceptance of the yoke of the mitzvot. The third is “Va-yomer” (“And He said”; Bamidbar 15:37-41), within which there is a command to remember the mitzvot via the mitzva of tzitzit, and at the end of which the Exodus from Egypt is mentioned.
The Sages arranged the Shema paragraph to precede Ve-haya im Shamo’a so that one will first accept the yoke of heaven and only then accept the yoke of the mitzvot. They also placed Ve-haya im Shamo’a, which presents a general command to keep all the mitzvot, including those performed during the day and those at night, before Va-yomer, which discusses the mitzva of tzitzit, performed only during the day (Berakhot 13a).
According to most Rishonim, the biblical commandment can be fulfilled by reciting just the “Shema Yisrael” verse, for about that verse Torah says (Devarim 6:6-7), “These words I am commanding you today must be in your heart…when you lie down and when you get up.” However, the Sages ordained the recitation of all three paragraphs. It can therefore be explained that the essence of the mitzva of Shema is indeed the acceptance of the yoke of heaven, which is why most Rishonim maintain that even one who only recites the first verse fulfills the biblical commandment. Yet, the more one enhances his acceptance of the yoke of heaven, the more completely he fulfills the biblical mitzva. Therefore, the Sages ordained the recitation of all three paragraphs, for within them are the fundamentals of faith, the acceptance of the yoke of the mitzvot, and the remembrance of all the mitzvot through the mitzva of tzitzit. Hence, in truth, we fulfill the biblical mitzva by reciting all three paragraphs (Peninei Halakha: Prayer, ch. 15 n. 1).