Why are the Edomim and the Egyptians permitted after two generations, whereas Amon and Mo'av are forbidden forever?
Rashi: Because whereas the former only harmed them physically (refer to 23:8:1:1), the latter caused them to sin, 1 which is worse. 2
Ramban: Because Edom are sons of the same father as us (Yitzchak), and because the Egyptians hosted our fathers in the time of famine, 3 honored our father Ya'akov and appointed one of his children viceroy.
This supports the Malbim, who says that also Amon were also liable for hiring Bil'am (Refer to 23:5:151:3). See Torah Temimah, note 40, who asks why the women of Edom and Egypt are then more stringent than those of Amon and Mo'av.
Rashi: Since killing a person takes him out of this world, whereas causing him to sin takes him out of the world to come.
Even though they their initial intention was for their own good. See Torah Temimah, citing Berachos, 63b, and note Bava Kama, 92b: And this is the source of the mantra 'Don't throw a clod of earth into the well from which you just drank'.
From when does one reckon the third generation?
Ramban (citing Yevamos, 78a): It is reckoned (not from the days of Moshe, but) from each Edomi or Egyptian who converted - his grandchildren are permitted.
What are the implications of (superfluous) words "Banim asher Yivaldu"?
Yevamos, 77b: Snce the Torah writes " ? asher Yivaldu" - attribiting the Isur to birth - there is no difference between the males and the females. 1
Yevamos, 75a: To teach us that if a Mitzris who is pregnant converts, she is a Mitzris Rishonah and when her baby is born he will be a Mitzri Sheini, and the same applies to the baby of a Mitzris Rishonah who marries a Mitzri Sheini. 2
Why does the Torah add the (otherwise superfluous) word "asher Yivaldu Lahem"?
Yevamos, 78a: To teach us that the third generation begins with the convert himself - and not with his son or daughter. 1
See Torah Temimah, note 38.
Why does the Torah need to insert both "Banim" and "Dor Shelishi"?
Yevamos, 75a: "Banim" implies that two sons, rendering the third son permitted - therefore the Torah writes "Dor Shelishi", to permit the third generation from the convert. Whereas "Dor Shelishi" inplies from Har Sinai - therefore the Torah writes "Banim", implyig the third son of the convert.
Why does the Torah insert the )otherwise superfluous) word "Yavo lahem ... "
Yevamos, 54b: We learn via a Gezeirah Shavah from "Lo Yavo Mamzer" from Chayvei La'avin (Mamzer) in Pasuk 3, that it incorporates in the Isur Ha'ara'ah (the first stage of intimacy, where the man's organ touches the woman's womb but does has not yet penetrated).
