What is the significance of the statement, "With this you will know that I am Hashem"?
Seforno: Pharaoh would now learn that Hashem (via the Name Havayah) manipulates the laws of nature, to destroy the Nile (which is indestructible by natural means). 1
Rashbam: It is the answer to Pharaoh's statement, "I do not know Hashem" (5:2). Now he was about to receive practical demonstration as to who Hashem was.
Riva: Pharaoh [made himself a god, and] said, "The river is mine, and I made myself" (Yechezkel 29:3). So Hashem informed him that he would now discover that Hashem, and not he, is G-d; when He will strike the river, and Pharaoh will be unable to save it.
Maharal: Refer to 7:17:1.1:1.
In the preceding verse (7:16), Moshe presented himself as the messenger of "Hashem, G-d of the Hebrews" (and similarly before Dever, Barad, and Arbeh). Why does our Pasuk emphasize the Specific Name (Havayah) - "With this you shall know that I am Hashem"?
Maharal (Gevuros Hashem Ch. 32, p. 123): All of the Makos ran against the order of nature, 1 but none were as wonderous as Makas Dam. 2 Other Makos were not an inherent change in nature - but rather an over-abundance, or super-concentration of one aspect of its balance. (For example, the Nile always produces frogs, only now they overran the land. Darkness is the absence of the usual light; darkness also exists to some degree on any cloudy day). Makas Dam, which changed nature, showed Hashem's truth; as such, our verse uses His Specific Name. (The preceding verse (7:16) refers back to when Moshe first appeared before Pharaoh (5:1-3). Moshe is saying, 'When I came before you at that time using this Name, you did not listen! Now, therefore, 'Koh Amar Hashem....''
The Specific Name is also used to introduce Tzefarde'a, Arov, Barad, and Makas Bechoros (refer to 7:26:152:1; 8:16:1:1; 9:29:3:3 and its note; 11:4:152:1).
Maharal (ibid. Ch. 56, p. 248): As the Hagadah Shel Pesach interprets, "Uv'Mofesim - This was the Dam;" only the plague of blood was an absolute change in nature.
Why did Hashem begin the plagues with the turning of the water into blood?
Rashi: Because, due to the fact that there is no rain in Egypt and they rely totally on the Nile overflowing its banks for their water supply, the Egyptians worship the Nile. So Hashem struck their deity first, before striking them. 1
Oznayim la'Torah #1: It was a punishment for the blood of the Jewish babies who were thrown into the River. 2
Oznayim la'Torah #2: Refer to 7:21:1:2*.
Maharal: Refer to 7:14:4.
Maharal (Gevuros Hashem Ch. 32, p. 122): Avodah Zarah is the idolaters' strength; it is the cause of their sin. One must strike at the source first!
It seems to me that the punishment for drowning the Jewish babies was the drowning of the Egyptians at the Yam-Suf - as the Ramban will explain; and that the water turning into blood was the punishment for slaughtering the newborn babies and bathing in their blood. In any event, the ten plagues were basically a punishment for Pharaoh's refusal to send Yisrael out, as we will see later.
Why did Hashem tell Moshe to say, "... with the staff in my hand"?
Hadar Zekenim: Because Pharaoh was afraid of the staff.
Oznayim la'Torah #1: Based on the fact that the magicians used two methods when performing their magic - Ma'aseh Sheidim (demons) and Ma'aseh Keshafim (witchcraft), Hashem proved to them that what Moshe was doing was based on neither of them - because, having changed from Aharon's staff to Moshe's, 1 Aharon demonstrated that he was not particular as to which implement he used, in which case it could not be Ma'aseh Sheidim; 2 Neither can it have been witchcraft, since Makas Dam occurred with water, and the moment anything that is bewitched touches water, it disintegrates.
Oznayim la'Torah #2: Hashem was initiating Moshe's staff, a staff of anger, with which they would chastise Pharaoh according to his wickedness.
