Why, by some of the plagues did Moshe warn Pharaoh, yet by others, he did not?
Ramban (to 8:15): He warned him for plagues that could cause death - including Dever, in which the animals died. 1
Rashbam and Seforno (to 8:12): He warned him before two out of every three plagues 2 - for Dam and Tzefarde'a, for Arov and Dever, and for Barad and Arbeh; but not for every third one - Kinim, Shechin and Choshech. 3
Maharal (Gevuros Hashem Ch. 57, p. 252): Maharal writes at length about the grouping of the Makos as 'De'tzach, A'dash Be'achav;' see 7:14:8. As for why the third Makah in each set was not preceded by a warning, refer to 7:14:8:2, and to 7:14:8:3*.
Did the frogs cause widespread death among the Egyptians? This Ramban supports the opinion that defines "Tzefarde'a" as crocodiles - which kill. Refer to 7:27:1:2. (Also see the Pasuk in Tehilim, "He dispatched against them Arov, which devoured them; and Tzefarde'a, which destroyed them" (Tehilim 78:45 - CS.) But refer to 7:15:2:1**, however, where the Ramban seems to contradict himself.
Seforno: As is hinted in Iyov 33:29.
Da'as Zekenim and Hadar Zekenim (to Pasuk 25): This is comparable to the Gemara in Sanhedrin 81b, which rules that someone who was warned twice before receiving Malkos for transgressions; the third time he is put in a cell to die, without a warning. Because the ten plagues came in three independent groups - as the Siman 'De'tzach, A'dash Be'achav' indicates - representing three on the ground, three above the ground and three in the sky; or, as the Seforno explains, the first group took effect on the two elements water and earth; the second group on living creatures; and the third group, on the air. (Makas Bechoros is not really part of the third group (Be'achav), since, unlike all the others, it came, not as a warning to Pharaoh to relent, but as a punishment for failing to do so - Ramban and Seforno. It is only mentioned together with it for convenience.)
"Bo El Pharaoh" implies that Moshe should appear before him at the palace. Why do the Makos follow a set pattern? In the first out of each set of three Makos, Moshe confronted Pharaoh at the Nile; in the second, at the palace; and the third had no warning at all.
Based on what the Ramban wrote in 7:15:2:1**, why did Hashem not say "Bo El Pharaoh" even by Makas Kinim and Makas Shechin - seeing as they did not cause death?
Ramban (to 8:15): Because for lice, 1 Aharon had to strike the ground - and the floors of Pharaoh's palace were presumably made of marble. For boils, Moshe had to throw the soot up toward the sky, in which case, he would have had to meet Pharaoh outside in the garden or courtyard.
Why did the Ramban ask about lice, but not about darkness, which does not cause death either, yet Hashem did not say 'Bo El Pharaoh'? (PF). In fact, since Hashem instructed Moshe to raise his hand toward the sky - see 10:21 - the same answer that the Ramban gives for the plague of boils will also apply to the plague of darkness.
Prior to Makas Dam (and other Makos), Moshe addressed Pharaoh in the name of "Hashem, G-d of the Ivrim" (7:16). Why is it significant that before this Makah, Moshe is to use the Specific Name (Havayah) [only]?
Maharal (Gevuros Hashem Ch. 32, p. 122): The warning before Makas Tzefarde'a came with the Name Havayah, because this Makah publicized His Name. 1 Nature usually operates at the time and location in which Hashem has programmed it; whereas in this plague, Moshe offers to Pharaoh, "For when shall I request that the plague be removed?" (8:5). This demonstrated clearly that Hashem is unique, and above all else. 2
Refer to 7:17:1.1:1 regarding Makas Dam; to 8:16:1:1 regarding Arov; also Barad (9:29:3:3 and its note); and Makas Bechoros (11:4:152:1).
Maharal: The Pasuk makes it clear that the goal was to publicize Hashem's Name. Moshe responds to Pharaoh, "As you say - so that you will know, that there is none like Hashem our G-d!" (8:6).
