Laws of the "Shema" - Part 1
Based on the verse, “You shall teach them to your children and you shall speak of them (the words of the Shema)…When you lie down and when you rise” (Dvarim 6:7), the Rabbis explain that it is a positive commandment to recite each day, both morning and evening, the ‘Shema’.
According to the Sefer HaChinuch it is only a Biblical command to recite the first verse, which begins with the word ‘Shema’ (Deverim 6:4). He writes that the remainder of the three parshiot that we say — “Ve’ahavta”, “Vehaya” and “Vayomer”— are of Rabbinical origin. Others maintain that the reciting of the entire first parsha is Biblical (Rabbeinu Yona, while others maintain that the first two parshiot are of Biblical origin (See Minchat Chinuch and Piskei Teshuvot).
The time of the evening Shema begins with the appearance of three small stars, and one should ideally say it at this time. The Rabbis said that one should recite the Shema before chatzot (midnight). However, if one delayed, he can say it until dawn since on a Biblical level its time extends until then. If one did not recite Shema before dawn he can no longer fulfill his obligation unless it was because of circumstances beyond his control — for example, if he was intoxicated, sick or the like (Shulchan Aruch 235:1,3,4).
The time for the morning Shema begins from the time one can recognize someone with whom he is slightly familiar from a distance of four amot (about six feet). The Piskei Teshuvot explains this time as fifty minutes before sunrise. Its time extends until the end of the third hour, which is a fourth of the day. The best way to fulfill the mitzvah of the morning Shema is like the Vatikin (humble people who treasured the mitzvot — Rashi). They would complete the Shema together with its blessings just before sunrise, and pray the Shemoneh Esrei at sunrise. One who is able to accomplish this merits a great reward (Shulchan Aruch 58:1).
The purpose of the mitzvah of Shema is in order to accept upon oneself