[Grammar] can't & must not

[beachboy]

John always walks to work. He can't have a car.

Does "he must not have a car" convey the same idea? If so, which one is more common in everyday English? Can I use the contraction "mustn't in this case?

 

[GoesStation]

"Can't" means something is preventing him from having a car - a legal judgment, for example. "Must not" (which can't be contracted in this meaning) means the speaker thinks that John doesn't have a car.

 

[Raymott]

In AusE, in the context of "John always walks to work. He mustn't have a car", "mustn't" would mean that it must be the case that he doesn't have a car.
In the context of "John is on parole. He must report twice a week to the police, and he mustn't have a car," the meaning is that he is not permitted to have a car. So the context matters.