In the introduction to this week's parasha, Hashem promises Avram, "Va-avarkha me'varakhekha, um-kalelkha a'or, ve-nivrekehu ve-kha kol mishpechot ha-adama" (Bereishit 12:3). This is commonly translated as "I will bless those who bless you, and he who curses you, I will curse, and through you all the families of the land will be blessed." This interpretation faces two problems, one linguistic and the other logical. First, in the pasuk's second clause, why does Hashem use two different words to mean "curse?" Instead of "um-kalelkcha a'or," wouldn't it be clearer (and more parallel to the composition of the first phrase of the pasuk, "va-avarkcha me'varakhekha") to say "um-kalelkha akaleil?" Second, if Hashem curses anyone who curses Avram, then how will all the families of the land be blessed? Won't those who curse Avram be excluded from the blessing?
The Gra answers these two questions by reinterpreting the word "a'or." He says that it comes not from the root "aleph-reish-hey," meaning "to curse," but rather from the root "aleph-vav-reish," meaning "light." Hashem promises Avram that those who curse him will be "shown the light," or enlightened. Even Avram's enemies will eventually see his true greatness and will thereby come to bless him. Ultimately, all the families of the land will bless Avram and his descendants, and they will all be blessed for having done so.