Seeing as the Heaven was already created on the first day (1:1), why does the Torah see fit to add here (on the second day) that Hashem commanded, "Yehi Raki'a"?
Rashi: Although it was created on the first day, it was still wet and therefore incomplete; so Hashem commanded it to dry up and strengthen itself.
Ramban: Based on his explanation in to1:1, 1 that (apart from the light) Hashem only formed 'Bohu' (indefinable matter), he explains that Hashem now ordered the matter of the Shamayim that He initially created, to take on the shape of a flat expanse (the word 'Roka' means to beat flat) that divides between the upper water and the lower water. 2
Rashbam: The Rashbam essentially agrees with the Ramban's explanation (see Answer 2). He adds that up until now, the water filled the entire space between the earth and the upper Heavens, and now, Hashem split the water, by placing the sky in the middle - from east to west and from north to south.
Yerushalmi Berachos, 1:1 #1 (according to Rav): It means that the sky should strengthen itself, become dry and grow a crust.
Yerushalmi Berachos, 1:1 #2 (according to R. Yehudah ben Pazi): It means that it should become a beaten (expanse). 3
The Ramban bases his explanation on the second explanation cited in a Midrash (the first explanation there is the one cited by Rashi).
The Rashbam explains that 'Raki'a' is another way of saying 'Meruka' (like 'Ashir' and 'Me'ushar').
Like the Pasuk writes in Parshas Pekudei, "vaY'rake'u Es Pachei ha'Zahav" (Shemos 39:3).
Why does the Torah write that the sky was situated "in the midst of the water"?
Rashi 1 : It comes to teach us that since the sky divided between the upper waters and the lower water, it must have been held in place by the 'Command of the King.' 2
See also Targum Yonasan.
The Ramban explains that he declines to offer an explanation, as it is forbidden to delve into these matters. On a number of occasions the Ramban intimates that even when he does get involved in esoterics, any reader who thinks that he understands it will only be fooling himself.
Seeing as the upper-waters were located in the vicinity of the Mal'achim and the Kisei ha'Kavod, what did Hashem do to compensate the lower-waters?
This explains the covenant that Hashem made with the lower-waters. Refer to Vayikra 2:13:1:1 .
Why did Hashem divide the water into two?
Oznayim la'Torah (citing the Pesikta Zutr'sa): Because He foresaw that the Dor ha'Mabul would sin and be worthy of destruction, and He prepared the upper waters and the lower waters to punish them. 1
QUESTIONS ON RASHI
Rashi writes: "'b'Soch' - ... in between the waters." Why doesn't Rashi interpret literally, that the firmament (Raki'a) was inside the waters?
Gur Aryeh #1: The next verse (1:7) says, "... the waters that were above [i.e. distant from] 1 the firmament." If so, "b'Soch" does not mean 'inside the waters,' but rather, 'in the middle, equidistant.'
Gur Aryeh #2: Rashi writes that the firmament had already been inside the waters, since day one. Now, on day two, it was to become a separation in between the upper and lower waters, in the middle.
