What did He mean when He told Moshe that He had hardened the hearts of Pharaoh and his servants in order to place His signs in their midst?
Seforno: He meant that the plagues would give at least some Egyptians an opportunity to do Teshuvah.
Why did Hashem now inform Moshe that he had hardened Pharaoh's heart?
Seforno: When Pharaoh declared "Hashem ha'Tzadik... !", Moshe assumed that, even though he had not relented out of fear of Hashem, 1 he would capitulate out of fear of the Makos. When he saw that this was not the case, he figured that his warnings were in vain. So Hashem told him that it was He who had hardened his heart already from the plague of boils, in order to send more plagues on Egypt - a. to encourage at least some Egyptians to do Teshuvah, and b. so that Yisrael should relate to future generations Hashem's greatness and goodness. 2
Moshav Zekenim: After Pharaoh admitted "Hashem ha'Tzadik", it says that he increased to sin. Therefore, it says "I hardened his heart."
Hashem instructed Moshe to come before Pharaoh, but does not inform us why?
Ramban (on Pasuk 2): It was to pass on the message spelt out in Pesukim 3-6.
Why does the Pasuk not tell us Hashem's instructions to Moshe?
Bechor Shor, Ramban (on Pasuk 2): It is the way of the Torah to tell us half the story and to take the other half for granted. 1 Here the Torah recorded what Moshe told Pharaoh, in order to bring the response of Pharaoh and his servants.
Riva, citing R"M mi'Kutzi: The hail did not finish off all the vegetation, because Pharaoh was somewhat humbled. When he returned to sin, Moshe told him that if he does not let Yisrael go, Arbeh will finish what the hail did not.
Rosh, Hadar Zekenim, Da'as Zekenim (4): Moshe was able to derive Arbeh from a Gezeirah Shavah "Tesaper-Sipru" from Yo'el. 2 Hashem taught the Nevi'im to Moshe.
Moshav Zekenim: "Ososai" alludes to the last four Makos.
Moshav Zekenim citing Bechor Shor: The initials of the Makos were written on his staff; he understood that Aleph is for Arbeh. 3
Moshav Zekenim: This is wrong. Yo'el was not written until long after Moshe! (Why would Hashem teach the Nevi'im to Moshe before Matan Torah, and before teaching him the Chumash?! Why would Moshe agree to send Meraglim if he knew (Yehoshua 14:2) "Himisu Es Lev ha'Am"? And why did Moshe pray to enter Eretz Yisrael, and fear Og, and need to stipulate that Bnei Gad and Bnei Reuven will cross the Yarden to fight? These outcomes are all clear from Yehoshua 1:2, 2:10, and 4:12! Perhaps Hashem withheld certain parts of the Nevi'im and did not teach them to Moshe, or made him forget them. We must say similarly about Vayikra Rabah 22:1, which says that Moshe received on Sinai everything that every Chacham will teach. We find that Moshe had Sefekos afterwards, e.g. killing the Mekalel and the Mekoshesh, Pesach Sheni, Benos Tzelafchad
Why does it say here "Bo El Pharaoh", and elsewhere, it does not (rather, "Lech")?
Refer to 7:26:151:1,2.
Since Hashem hardened the heart of Pharaoh, what was his sin? (It says "va'Yosef Lachato" (9:34).)
Hadar Zekenim: Hashem hardened his heart only after Pharaoh hardened his heart by himself three 1 times.
The Torah says that Pharaoh hardened his heart after Aharon's staff swallowed the magicians' staffs, and after each of the first five Makos! Three times it says "va'Yechezak", and three times it says "Hichbid". (However, Hadar Zekenim seems to use these interchangeably
It says "I hardened the hearts of Pharaoh and his servants", but his servants told him to let Yisrael go!
Moshav Zekenim: They said to send the men, i.e. but not the women and children.
Rashi writes 'Shisi
Moshav Zekenim #1: One might have explained 'I will put through an intermediary.' Rather, Hashem Himself will put.
This is difficult, for Hashem is speaking to Moshe! Also, how can a command follow the word "l'Ma'an" (in order that)? (PF)