What is the significance of the fact that Esav married when he was forty?
Rashi: Tehilim 80:14 compares the nation of Esav to a pig, whose feet resemble those of a Kosher animal, but (like Tamei animals) it does not chew its cud. So what does it do? When it lies down, it sticks out its feet, as if to announce 'See, I'm Kosher!' For forty years, Esav took women from their husbands and afflicted them. After he turned forty, he decided that he would follow in the footsteps of his father - to marry at forty. 1
Rashbam: Bearing in mind that Yitzchak was sixty when Rivkah bore Yaakov and Esav, it means that he was 100 when Esav married. Consequently, this serves as an introduction to the following Pasuk, "And it was when Yitzchak became old..." (27:1).
Esav's Kena'anite wives were called Aholivamah and Adah (36:2). Why does the Torah call them here Yehudis and Basmas, respectively?
Rashi (to Bereishis 36:2): Esav called Aholivamah by the name of 'Yehudis,' to trick his father into believing that she rejected idolatry. Adah was called Basmas 1 (like Besamim, i.e. spices) for she burned incense to idolatry).
Ramban (to 36:2, citing Ibn Ezra): She had two names.
Ramban (to 36:2): Esav's first two wives died (perhaps for paining Yitzchak and Rivkah), and he remarried.
Here, Esav's Kena'anite wives are called Bas Be'eri and Bas Eilon, respectively. Below (Bereishis 36:2), it says that they were Adah Bas Eilon and Aholivamah Bas Anah / Tziv'on!
Moshav Zekenim: Anah was Ahalivamah's mother. 1
Ramban (36:2, citing Ibn Ezra): Her father had two names.
Ramban (36:2): Esav's first two wives died. He married Basmas' sister Adah bas Eilon, and Aholivamah bas Anah, who had no relation to Be'eri.
Presumably, it mentions her mother, for Anah (and hence her daughter) was like a Mamzeres (although in Halachah, Mamzer does not apply to Nochrim). Just before this, Moshav Zekenim cited the Midrash that HaSh-m arranges for Mamzerim to marry each other. (PF)
Why didn't Yitzchak command Esav outright not to marry a Kena'anis?
Ha'amek Davar: This shows that Yitzchak understood that "b'Yitzchak Yikarei Lecha Zara" (21:12) refers to Yaakov, but Esav is not considered Zera Avraham. He thought that he is a good person, just not part of the chosen nation. Malbim - Before Yitzchak gave 'Birkas Avraham' to Yaakov, he commanded him not to marry a Kena'anis (28: 1,4).
QUESTIONS ON RASHI
Rashi writes: "When [Esav] was forty years old, he said, 'Father married at 40 years old; I shall as well.'" Why expound this way?
Gur Aryeh: In other instances, the Torah does not record how old a person was when he married (not even regarding Yaakov). This shows that there was a reason that Esav married specifically at forty.
Rashi writes: "Father married at 40 years old." Why did Yitzchak marry at specifically this age?
Rashi writes: "'Father married at 40 years old; I shall as well." Why did Esav imitate Yitzchak in this matter?
Age 40 represents Shleimus and Din. On a deeper level, this means that Esav pretended to be an extension of Yitzchak's trait of justice; but in fact, he personified robbery and violence. 1
In other words, Yitzchak had perfected his Midah of Din, whereas Esav was the Kelipah (the outer shell, the negative aspect ) of Din - robbery and violence. (EK)