1 is correct: implicit reference to a particular car eg. the family's car.
2 is correct
3 is correct: eg. the police is suspecting someone to sell stolen cars and asks him where he keeps them.
4 is incorrect AFAIK
5 is correct.
6 is incorrect: not countable
7 is incomplete: what stuff? It needs to be qualified.
8 is incorrect
Teachers to pop in if I'm wrong,
- For Teachers