Why does the Torah mention specifically now the distinction between Goshen (where Yisrael lived) and the rest of Egypt?
Rashbam, R. Bachye, Riva and Hadar Zekenim (in Pasuk 17): Because, as opposed to the previous plagues (which were confined to one area), the wild beasts roamed around, 1 and there was no logical reason for them to avoid entering Goshen 2 (Hence Hashem added "asher Ami Omeid alehah" ? R. Bachye).
Moshav Zekenim: Because, since Arov and Dever came by themselves, not through Moshe's staff, Par'oh suspected that they came by chance, and not because he was detaining Yisrael. Therefore Moshe pointed out that they will not affect Yisrael, to demonstrate that it came due to Par'oh's sins. 3
Oznayim la'Torah (to 8:17): Following Kinim, which affected Yisrael as well, 4 Hashem informed Par'oh that this would not be the case regarding Arov, where He would make a clear distinction between Yisrael and the Egyptians. 5
If anything, one would have expected the plague to be more rampant there, since that was where the sheep were.
Rashbam: And it is based on the same logic that the Torah uses the same expression regarding the plague of Dever (pestilence).
By the same token, when Arov left through Moshe's Tefilah, Par'oh knew that it was from Hashem. Dever did not leave, therefore Par'oh sent to see if Yisrael's animals had also died.
See Oznayim la'Torah.
Bearing in mind that when Yisrael came down to Egypt, Yisrael said "Yeishvu... Avadecha b'Eretz Goshen" (Bereishis 47:4); and Pharaoh reiterated, "Yeishvu b'Eretz Goshen" (Bereishis 47:6), why did Hashem say here "Asher Ami Omeid Alehah"?
Oznayim la'Torah: Because, whereas when they came down to Egypt, they came to remain for an extended period of time; now the year of their departure was imminent, and the people were ready to leave. This is hinted in the word "Omeid."
What is the significance of the phrase, "Ki Ani Hashem b'Kerev ha'Aretz"- "'I am Hashem, in the midst of the earth"?
Rashi, Ramban #1 and R. Bachye #1: Even though Hashem's Sh'chinah dwells in Heaven, His Providence ('Hashgachah') is effective on earth. (to counter the argument of Par'oh and those who say that, "the thick clouds block Him so He cannot see" ? Ramban). 1
Ramban #2 (according to Kabalah) and R. Bachye #2: It is equivalent to "ki Sh'mi be'Kirbo". 2
Ramban and R. Bachye (in 9:14): Later, before removing the plague of Barad, Hashem will say to Par'oh "Lema'an Teida ki la'Hashem ha'Aretz" (9:29) (since He created it out of nothing ? 'Chidush'). And before bringing it on, He said to him, "Lema'an Teida ki Ein Kamoni be'Chol ha'Aretz" (9:14) to teach Par'oh that He rules over the world and no-one can stop Him from doing what He wants ? 'Yecholes'). All of these are principles that the Egyptians denied or doubted. In fact, they form the basis of Emunah of the entire Torah.
See R. Chavel's footnotes on R. Bachye.
What is the significance of Hashem's statement, "Lema'an Teda [Ki Ani Hashem...] - so that you will know"?
Hadar Zekenim: Pharaoh had said, 'I do not know Hashem' (5:2); so Hashem told him that with this Makah you will begin to know Me.
Ramban: Refer to 8:18:2:1 and its note.
QUESTIONS ON RASHI
Rashi writes: "V'Hifleisi - [I.e.] 'I shall separate, make it distinct' ... as in [the verse], 'Lo Nifleis Hi' (Devarim 30:11)." But the words are not identical! The root of "Hifleisi" (in our verse) is Pey-Lamed-Hei, whereas the root of "Nifleis" is Pey-Lamed-Alef?
Gur Aryeh: When the third letter of a root is either a Hei or an Alef, it falls out at times (known as Nachei Lamed-Hei / Lamed-Alef). Rashi's approach is that since the roots' essential two letters are the same, the meaning will also be the same (or very similar), and we may derive one from the other. 1
Also refer to 7:1:151.2:1 and 9:31:0.1:1, and their notes.
Rashi writes: "'I am Hashem in the midst of the earth' - Although My Shechinah is in Heaven, My decree is fulfilled on earth." How did Makas Arov begin to teach Pharaoh this lesson?
