Seforno: It means that Yisrael should reflect ... .

Targum Yonasan: With reference to the sons of Ya'akov, who, in answer to their fathers fear that the there was a (spiritual) defect among them, declared in unison "Listen Yisrael our father, Hashem is our G-d ... "! ¹

B'rachos, 15a: It implies that, when reciting the Sh'ma, one is obligated to hear what one is saying.

B'rachos, 13a: It implies that as long as one hears what one is saying, one can recite the Sh'ma in any language. ²

B'rachos, 16a: We learn that the first Pasuk of Sh'ma ¹ requires Kavanah from a Gezeirah Shavah "Sh'ma" and "Haskeis u'Sh'ma" ² in Ki Savo, 27:9.

Hashem means 'the Omnipotent G-d', ¹ whereas 'Elokim" means 'the G-d who judges and supervises our deeds, and who rewards and punishes us'. ²

Chulin, 91b: It teaches us that Yisrael are more beloved by Hashem than the angels. ¹

Ramban: "Sh'ma Yisrael ... " is actually an elaboration of the first of the Aseres ha'Dibros - the Mitzvah of "Anochi" (declaring the Oneness of G-d).

Seforno: It implies that Hashem

Rashi: It means that Hashem, ¹ who is currently our G-d (Elokeinu), but not of the Nochrim, will eventually become One (because also they will recognize Him). ²

Da'as Zekenim and Hadar Zekenim (in 5:1): Had it said only 'Sh'ma Yisrael Hashem Echad', every nation would have said that their idolatry is one. Now that it says "Hashem Elokeinu", it is known that it is discussing Elokei Yisrael. Had it said Hashem Elokeinu ³ Echad, this would imply that Elokeinu is one of the gods, and all the more so if it would say Elokeinu Echad. Now that it says twice "Hashem Elokeinu Hashem", it means that Hashem, who is our Master, is one in Lordship, and there is none other than Him.

Oznayim la'Torah: It means that. irrespective of whether Hakadosh-Baruch-Hu exercises Midas Rachamim ("Hashem") or Midas ha'Din ("Elokeinu"), "Hashem Echad", it is always Rachamim - because even His Midas ha'Din is for our good, like a father to a son.

Ramban: In order to include himself in the Mitzvah of declaring Hashem One. ¹

Targum Yonasan: This was first said in a dialogue between Ya'akov and his sons, where, in reply to his query as to whether their Emunah was complete, they exclaimed in unison 'Sh'ma Yisrael Avinu, Hashem Elokeinu ... !' ² 'Listen Yisrael our father ... !'

Oznayim la'Torah: Because the Torah is presenting this Pasuk as if it was being said by Yisrael - just as it was said the first time by the sons of Ya'akov ³ - as in answer #2.

Rashi (in B'rachos, 13b): It means that Hashem is One ¹ in Heaven, on earth and in all directions. ²

Seforno: It means that there is nothing that can compare to the existence of Hashem, who created existence when there was nothingness. Consequently, He is detached from this (finite) world, from the world of the planets, from the world of angels, and He exists Alone in the fourth world. ³

Rashbam: It means that we will serve Hashem, who alone is our G-d, ⁴ and under no circumstances will we combine any other deity with Him, even to divine whilst still believing that there is no G-d other than Hashem. ⁵

Yerushalmi B'rachos, 1:5: It implies that Evadim, who have another master besides Hashem, are Peturim from reciting the Sh'ma. ⁶

Oznayim la'Torah: As opposed to the children of Avraham and of Yitzchak, who split up into different nations, the children of Ya'akov all united into one natqion serving the One G-d. ⁷

Ramban #1: "Sh'ma" is coming to elaborate on the Mitzvah of "Anochi", which is the unification of Hashem's holy Name.

Ramban #2 (in Yisro) ¹ : "Anochi" is the Mitzvah of Kabalas Ol Malchus Shamayim, whereas "Sh'ma" is that of the unification of Hashem's holy Name.

Seforno: To hint that we should open our eyes and think deeply about the Oneness of Hashem. ¹

Ba'al ha'Turim #1: Because Hashem gave the Torah which has seventy names and can be explained in seventy ways to Yisrael, who also have seventy names. ²

Ba'al ha'Turim #2: Refer to 6:4:8:5.

Perhaps it is in case people switch the 'Ayin' for an 'Alef' ³ , and it will sound like 'Shema' (perhaps), Chas ve'Shalom. (PF)

Ramban (according to Kabalah): It hints at the great Name of Hashem. ¹

Seforno: Because Hashem exists alone in the fourth world. ²

Ba'al ha'Turim #1: To teach us that we should declare Hashem King over Heaven and earth and over all four directions.

Ba'al ha'Turim #2 (citing Shir ha'Shirim Rabah 5:11): To avoid confusing it with a 'Reish' - reading it 'Acher' (Chas ve'Shalom). ³

Ba'al ha'Turim #3: In conjunction with the large 'Ayin' in 'Echad", it spells 'Eid' - a hint that Yisrael testify to Hashem's Oneness. ⁴

Refer to Sh'mos 32:7:3:2.