Who were the Nefilim? When did they live?
Rashi #1: Giants who lived in the time of Enosh and Kayin, who 'fell' (caused their own deaths), and took down the whole world with them. 1
Ramban (citing Ibn Ezra and Radak): Men who caused the hearts of those who saw them to drop.
Ramban: Refer to 6:4:3:3 .
Targum Yonasan: Shamchaza'i and Uzi'el (or Aza'el), who fell from heaven... . 4
Gur Aryeh: Why the second Derashah? Nefel, or (pl.) Nefalim, would mean that they themselves sinned and fell. The extra [first] Yud, "Nefilim", adds the meaning that they caused others to sin, causing their downfall along with them.
The people from which Og was the sole survivor.
See also Rashi in Devarim 2:11.
What is "ba'Yamim ha'Heim" referring to?
Rashi: It refers to the generations of Enosh 1 and of the B'nei Kayin - who were being wiped out due to the curse.
What are the connotations of "ve'Gam Acharei-Chein"?
Rashi: Even after they saw how in the generation of Enosh, one third of the world was drowned by a flood; they persisted in their evil ways and did not do Teshuvah.
Ramban (quoting the Ibn Ezra): These giants lived even after the Flood - with reference, either to the wives of Noach's sons, who were their descendants and who resembled them, or to Og, Melech ha'Bashan who, according to the Midrash, survived the Flood.
Seforno: The Nefilim were in the land when Hashem gave them a hundred and twenty years to do Teshuvah, and also after that - when the time elapsed, and they had not done Teshuvah.
Oznayim la'Torah: It refers to people like Nimrod, who continued with the legacy of the Nefilim, who made themselves out to be deities. 1
Why in connection with the B'nei ha'Elohim, did the Torah write "Va'Yikchu lahem Nashim ... ", and here "Asher Yavo'u B'nei ha'Elohim el B'nos ha'Adam"?
Oznayim la'Torah (based on the Targum Yonasan's distinction between the B'nei ha'Elohim and the Nefilim): This marks the distinction between the two groups of sinners. After the human sinners began stealing wives from wherever they chose - even against the wishes of the women and even if they were already married, the angels went one step further, and proceeded to indiscrimanently grab women and rape them. 1
See Oznayim la'Torah, who elaborates at great length and who answers many questions according to this explanation.
Who were the "Anshei ha'Shem"?
Rashi #1: They were the people specified by the Pasuk earlier in 4:18 - 'Irad, Mechuya'el and Mesusha'el,' whose names suggest destruction (because they were blotted out and destroyed). 1
Rashi #2 and Targum Yonasan: "Anshei ha'Shem" means 'men of desolation (from the word 'Shemamah').
Ramban #1: The 'Bnei ha'Elohim' were super-human-beings - with reference to Adam, Sheis and Enosh, who were all created in the image of Hashem. Starting from Keinan (Enosh's son) they began to practice idolatry and subsequently became weaker, though they were still stronger than everybody else (Kayin's children). They, using strong-arm tactics, were the ones who were guilty of sinning in the way described by the Pasuk. And the Pasuk says about them that, people who saw mighty men after the flood would point out that there had been men far mightier and more awesome than them.
Ramban #2 (citing Pirkei d'R. Eliezer and the Gemara in Yoma 67b): They were angels who fell from heaven. 2
Oznayim la'Torah; They are the Mal'achim -the Nefilim - who declared themselves deities. 3
QUESTIONS ON RASHI
Rashi writes: "'In those days' - In the generation of Enosh, and of Kayin's descendants." What happened at that time?
Gur Aryeh: The descendants of Kayin were wiped out by a flood, because of the decree, "[Hevel] shall be avenged after seven [generations]" (4:15). 1 In the days of Enosh, mankind began to worship idols (Rashi to 4:26).
See Rashi to 4:24; refer to 4:24:2.5:1.
Rashi writes: "... They would give birth to giants, like [their fathers]." How is this derived from the verse?
Gur Aryeh: The word "Asher" in this Pasuk does not mean "when," but rather, "because." Why were the giant Nefilim still around? Because when the "Bnei ha'Elohim" had children though the "Benos ha'Adam," they were born giant like their fathers.
Rashi writes: "'Men of name' - i.e. their names [in the verses] mean destruction." Perhaps it simply means famous or important men?
Gur Aryeh #1: The Torah would not call the wicked famous or important; 1 rather it means the descendants of Kayin who were named earlier (4:18). They are called "men of name" because their names hint at the destruction that would befall them.
Gur Aryeh #2: Just as merits can earn a person the title "Ba'al Shem" [meaning that his merits are part of his essence, and not just incidental]; so too were these men headed for destruction in their essence, as indicated by their names.
