What did Moshe mean when he said to Par'oh "Hispa'er alai"?
Rashi, Targum Onkelos and Targum Yonasan: He was issuing Par'oh a challenge ('Pride yourself - boast over me') - to ask Moshe to do something that he would not be able to achieve. 1
Rashbam: He was telling Par'oh that he would carry out his request, irrespective of how big (difficult) it would be. 2
Oznayim la'Torah: He gave Par'oh three proofs that the plagues were not based on witchcraft
What did he mean when he said "le'Masai Atir L'cha ... "? Why the extra 'Lamed'?
Rashi: He was asking him (not when he wanted him to Daven - that he would do immediately, but) when he wanted the plague to be removed.
Ramban: As a matter of principle, the moment Moshe Davened to Hashem, the plague came to an abrupt end, and the extra 'Lamed' is not significant. 1
Rashbam: Since the frogs were unlikely to all die in one go, he was asking him by when he wanted all the frogs to have died. 2
Seforno: As opposed to the witchcraft of the magicians, which lasts only for a limited time and then disintegrates, Hashem's miracles are not limited to a time-period. Consequently, Moshe told Par'oh that he should fix a final date for the plague and Hashem would comply. 3
Hadar Zekenim, Riva (in Pasuk 6): There is a P'sik (a break) between le'Masai and Atir. Moshe asked, when will you send out Yisrael? I will pray for you now.
What message is Hashem conveying to Par'oh in the latter half of the Pasuk?
Ramban: He was informing Par'oh that He would destroy the frogs from the land and that he need fear their return, even though some would remain in the Nile. To impress upon him that the plague was sent by Hashem, solely for the sake of Yisrael. 1
As the next Pasuk concludes "So that you should know that there is no-one like Hashem our G-d!"