"America is very far away from us."
Translation:Η Αμερική είναι πολύ μακριά από εμάς.
Actually, strong forms (εμάς) are in the accusative case. The weak form (μας/μάς) is supposedly in the genitive case (which is the same as the accusative, in this case, because "μας" doesn't change) and is not preceded by a preposition.
"Η Αμερική μάς είναι πολύ μακριά" is grammatically correct, albeit rare.