The Case filter is what prohibits the phonetic realization of a DP that recieves no grammatical Case. In languages such as Portuguese, people use impersonal constructions like "há/tem um carro na rua", meaning "there is a car in the street". The verb "haver" (and "ter", nowadays) is impersonal, so the subject is always expletive and null in this language (which is pro-drop).
The problem is: how does the DP "um carro" get its Case to pass the Case filter? It doesn't seem to be receiving accusative Case by the verb, I guess. What is happening in this kind of construction?