What did Yitzchak command Yaakov about marriage?
Malbim: He stipulated with a Lav against marrying a girl from Kena'an 1 (28:1). He gave him an Aseh to marry a daughter of Lavan, and a second Berachah [to have many children - 28:3].
Ha'amek Davar: He warned (forbade) him about marrying a girl from Kena'an, and advised him to go to Lavan to find a wife.
Ohr ha'Chayim (to 28:5): This is because Kena'an is cursed, and Kedushah cannot cling to what is cursed. Nations convert, but we never heard that a Kena'ani converted! The Torah said "Lo Sechayeh Kol Neshamah," for no soul from them will convert! (Kidushin 76b assumed that Uriyah ha'Chiti was a convert from Ches, but rejected that [perhaps] he merely lived in Eretz Chitim. However, Alshich (Shmuel II 12:1) and Pnei Yehoshua (Kidushin 76b) cite a Midrash that he was a Chiti convert! - PF)
QUESTIONS ON RASHI
Rashi writes: "[For] any word that requires a Lamed at the beginning, the verse [may instead] put a Hey at the end." Why isn't this the case for names of people as well?
Gur Aryeh: We never say 'Reuven came Shim'onah,' or the like. A final Hey indicates entering into, e.g. in this verse, '[enter] into the house of Besu'el.' 1 This can be said only of a location or city, not of a person.