Is 'the son of Robert' or 'the car of John' correct usage? Or is it essential to say "Robert's son"?
The first one can sound okay if you have a longer phrase.
Who is Peter?
He's the son of one of my old college friends.
If your listener already know's Robert and you want to talk about his son, then "The son of Robert" will sound very odd to most people where I live.
I have a hard time thinking of when "the car of John" would ever sound right.
EDIT: Oh, and welcome to Using English!
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.