What does the Torah mean when it writes that Aharon raised his staff (a seemingly superfluous statement)?
Ramban: It means that Aharon first waved the staff in all directions 1 before striking the water of the Nile.
Thereby ensuring that all the water would be stricken and not just the water of the Nile.
What is the significance of the fact that Aharon struck the water before the eyes of Pharaoh?
Rashbam: The fact that Aharon acted as Moshe's Shali'ach, raising his staff and striking it each time upon his command, boosted Moshe's esteem in the eyes of Pharaoh.
What is the law regarding a Tahor person who eats Tamei Kodshim?
Rashi: A Tahor person who eats Tamei Kodshim transgresses only a La'av, which we learn from "veha'Basar asher Yiga be'Chol Tamei". 1
See Pasuk 19.
Why does the Torah insert three Kerisos in connection with someone who eats Kodshim b'Tum'as ha'Guf (our verse, the next verse and 22:3)?
Rashi (Shevu'os, 7a): It was in the Klal and left, to teach that Kares is only for Kodshei Mizbe'ach; one is exempt for Kodshei Bedek ha'Bayis.
Riva, Moshav Zekenim (19): In Shevu'os 7a, [we conclude that] the third obligates for things that are not normally eaten, e.g. wood, Levonah,and Ketores. (There, R. Shimon learns from it that one is liable for inner Chata'os for Tum'ah, even though he holds that Pigul does not apply to them. - PF)
Rashi writes that the warning against eating Kodshim be'Tun'as ha'Guf is not explicit in the Torah. He himself (12:4) said that "Lo Siga" warns against eating Kodshim. This shows that it is explicit!
Moshav Zekenim left this difficult. Seemingly, also this is not explicit! (PF)
Rashi (Shevu'os 7a) writes that we expound Kerisos written about eating Kodshim b'Tum'as ha'Guf as a 'Klal, and something that left the Klal
Riva, Moshav Zekenim (from Menachos 55b): We do not expound Klal u'Ferat when they are distant from each other.