When is the earliest time in the evening when one can fulfill the Misva of reading the nighttime Shema?
This issue is subject to a debate among the Rishonim (Medieval Talmud scholars). Rashi (Rabbi Shlomo Yitzhaki, France, 1040-1105), commenting to the first page of Masechet Berachot, writes that communities who recite Arbit early, before sundown – as many communities do in the summertime nowadays – do not fulfill the Shema obligation with that recitation. They must therefore repeat the Shema after dark, and they fulfill the Misva through the recitation of the bedtime Shema. According to Rashi, then, one cannot fulfill the Misva of the evening Shema before Set Ha’kochavim (nightfall).
Rabbenu Tam (France, 1100-1171) disagreed. He claimed that just as one can fulfill the requirement of Arbit before sundown, one can fulfill the Shema obligation at that time, as well. This is also the position of the Ri (France, 12th century), who noted that the Gemara cites views allowing reciting Shema even before Set Ha’kochavim.
The Shulhan Aruch (Orah Haim 235; listen to audio recording for precise citation) writes that if one prays Arbit with a Minyan before sundown, he should recite Shema with the congregation but he does not fulfill his Shema obligation at that time, and he must therefore repeat Shema (without the accompanying Berachot) after dark. The Shulhan Aruch thus follows Rashi’s position, that the evening Shema must be recited after Set Ha’kochavim, and this is, indeed, the Halacha. This is why when congregations recite Arbit early – as is commonly done on Friday night during the summer months – the Rabbi announces after the prayer service that everyone should ensure to repeat Shema after dark.
When after dark should a person repeat the Shema?
Intuitively, we might assume that one should simply recite all three paragraphs of Shema at the time of the bedtime Shema before he goes to sleep, and have in mind to fulfill his Shema obligation at that point. However, the Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) writes that one should not rely on the bedtime Shema to fulfill the evening Shema obligation. People generally recite the bedtime Shema with the intention of protecting themselves from Mazikin (harmful spirits), and not for the Misva, and in order to fulfill the Shema obligation one must have in mind that he seeks to fulfill the Biblical command of Shema. And even if we argue that one fulfills the Misva without this intention, the Misva does require the serious acceptance of God’s kingship, and people generally recite the bedtime Shema casually, when they are not in the frame of mind to contemplate such weighty matters. Therefore, the Mishna Berura writes, one who recited Arbit early, and must therefore repeat the Shema, should make a point of reciting the evening Shema at some point before bedtime, rather than relying on the Keri’at Shema Al Ha’mita.
One might, at first glance, question the Mishna Berura’s ruling in light of Rashi’s comments cited earlier. Rashi wrote explicitly that one can fulfill the obligation of the evening Shema through the recitation of Keri’at Shema Al Ha’mita. If Rashi clearly allows relying on the bedtime Shema for the Misva, why did the Mishna Berura write that one should not rely on the bedtime Shema?
The answer becomes clear when we read Rashi’s comments more carefully. Rashi writes, “Therefore, we are obligated to recite it when it gets dark,” and then he proceeds to comment that one fulfills the Shema obligation through the recitation of Keri’at Shema Al Ha’mita. This implies that optimally one should recite Shema after nightfall, but if he did not, then he can fulfill the Misva through the bedtime Shema. Rashi did not allow relying optimally on the bedtime Shema for fulfilling the Misva, but rather mentioned that one can fulfill the Misva through the bedtime Shema if he had not recited it earlier. As the Mishna Berura wrote, it is preferable to recite it after dark rather than waiting until Keri’at Shema Al Ha’mita. In fact, Rashi comments may likely have been the source for the Mishna Berura’s ruling.
Summary: One cannot fulfill the Misva of the evening Shema before dark. Therefore, if one prays Arbit before dark, even though he recites Shema as part of Arbit, he must repeat Shema after dark. One should not rely on the Keri’at Shema Al Ha’mita recitation for this purpose.

Adopted from the class of Rabbi Eli Mansour