Why did Hashem ask Chavah what she did, even though He obviously knew already?
Seforno: To encourage her to do Teshuvah, since Hashem does not want the Rasha to die. 1
Since Chavah was not yet created when Adam was issued the warning not to eat from the Eitz ha'Da'as, why was she punished?
Ramban: Because she was part of Adam at that time, the warning automatically pertained to her as well.
Sanhedrin 38b: Chavah was created in the seventh hour. Hashem commanded not to eat later, in the ninth hour (the Torah is out of order).
Why did Hashem not ask her why she ate from the Tree, like He asked Adam?
Ramban: Because He was also taking her to task for causing Adam to sin 1 (like the snake). And in her answer, she only referred to the sin of eating, since that was the major sin.
From here we can learn, says the Ramban, that someone who causes somebody else to sin will receive his due punishment.
What does "ha'Nachash Hishi'ani" mean?
Rashi: It means 'the snake tricked me'.
Targum Yonasan: Just as earlier, in Pasuk 1, he translated the Torah's description of the snake "Arum" as 'wise to do evil', so too here, he explains 'he counseled me in his wisdom and tricked me in his wickedness'. 1
Rashi (Shabbos 146a): Hishi'ani is from the root of Nisu'in. The snake had Bi'ah with her (like the Gemara says there).
Peninim mi'Shulchan ha'Gra: It is an expression of Hisnas'us (haughtiness of heart, which leads man to investigate what he should not investigate.
It is important to remember that this defines the Yetzer ha'Ra perfectly, wise and wicked.
רש"י: הטעני: למה רש"י לא פירש מלשון הסתה?
גור אריה: רש"י לא פירש שאמרה הנחש הסית אותי, כי אין זו תשובה לחוטא באומרו פלוני הסיתני.