What did Moshe mean when he said to Pharaoh "Hispa'er alai"?
Rashi, Targum Onkelos and Targum Yonasan: He was issuing Pharaoh 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 Pharaoh that he would carry out his request, irrespective of how big (difficult) it would be. 2
What did he mean when he said "l'Masai Atir Lecha ... "? Why the extra 'Lamed'?
Rashi: He was asking him (not when he wanted him to pray - that he would do immediately, but) when he wanted the plague to be removed.
Ramban: As a matter of principle, the moment Moshe prayed 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 Pharaoh that he should fix a final date for the plague and Hashem would comply. 3
Hadar Zekenim, Riva (verse 6): There is a Pesik (break) between l'Masai and Atir. Moshe asked, when will you send Yisrael? I will pray for you [now].
What message is Hashem conveying to Pharaoh in the latter half of the Pasuk?
Ramban: He was informing Pharaoh 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!"