After issuing the warning about the plague of hail, why did Moshe advise the people to gather the animals and the people under cover?
Ramban: Because the objective of the Barad was to destroy the crops, not the animals and people. 1
Seforno: In order to save the slaves who shepherded the flocks. 2
Since the plague was due to begin only 'tomoriw', why did Moshe advise the Egyptians to to gather the animals and the people unnediately?
Oznayim la'Torah: presumably, he was worried about those who tended to the animals far from the town. 1
Oznayim la'Torah: As in the Mishnah in Beitzah, 40a.
Why did Hashem constantly divulge his plans to Par'oh, something that human kings don't do?
Rashi (in Iyov, 36:22): Unlike a human king, who needs to hide his plans from his enemies to prevent them from utilizing their knowledge to outwit him, Hashem knows that nobody can upset His plans, so He has no reason to conceal them; whereas by revealing them, He offered Par'oh a chance to do Teshuvah.
Why did Moshe warn to take the animals into the house before hail, but not before Dever?
Moshav Zekenim: If they took the animals in before Dever, nothing would die, whereas the hail would destroy the vegetation 1 in any case.
According to the Ramban (refer to 9:3:1:2), Dever killed also animals in the house.
Which was the main objective of Makas Barad. See Oznayim la'Torah, on Pasuk 25, DH 'Vayach ha'Barad.