If Pharaoh asked both Moshe and Aharon to pray for him ("Ha'tiru El Hashem"), why did only Moshe Daven?
Oznayim la'Torah: Because throughout the Makos, it was always Moshe who Davened to remove the Makos - even those that Aharon brought on.
Rashi (to 7:25) wrote that each Makah was in force for seven days. But if seven days had already elapsed, why did Moshe have to pray? And if seven days had not yet passed, then the Makah was not in force for its full duration!
Gur Aryeh (to 8:5): Even when a Makah was cut short by Moshe's Tefilah, Pharaoh lived in fear that it would return at any moment. 1 (In that sense, these Makos also lasted the full week. Refer to 7:25:1.3).
Maharal #1 (Gevuros Hashem beg. Ch. 33, p. 124): The Makah itself lasted seven days, yet its after-effects would have lingered far beyond, had Moshe not prayed. Moshe's prayer caused that the Arbeh that was already present would disappear. 2
Maharal #2 (ibid.): Each plague was to last for seven days - provided that Pharaoh did not agree to send the Bnei Yisrael free. On the occasions when Pharaoh said that he would free them, this completed the Makah. Moshe's prayer was then effective in removing its effects - even during the seven days. 3
It is unclear why Gur Aryeh (loc. cit.) mentions only Tzefarde'a, Arov, Barad and Choshech in this regard - omitting Arbeh. In Gevuros Hashem (see below), however, he references Arbeh as well.
Oznayim la'Torah (to 10:16): As well as any eggs they managed to lay, which would have caused additional waves of locusts - see 10:16:1:3.
But prior to Makas Arbeh (10:8), and during Makas Choshech (10:24), Pharaoh spoke about freeing the Bnei Yisrael - so why didn't this stop the Makah? However, at these times, Pharaoh would not agree to Moshe's terms, of freeing even the Bnei Yisrael's children and their livestock. (During Choshech, he failed even to ask Moshe to pray). That is why Arbeh began; and why Makas Choshech continued for its intended duration. (CS)
