en vs dans

Question

In an A1 focus test  I wrote"ton père est DANS la prison" because a prison is a physical enclosure. It was marked as incorrect. could you please explain why? Thanks.

Accepted Answer

This follows basically the same distinction as the examples using “classe” in the lesson. 
Prison is treated as an abstract noun when referred to generally. We have a similar distinction in English  - we wouldn’t use an article in the abstract situation.
 Le voleur est en prison/the thief is in prison (imprisoned or incarcerated)- is the example in the lesson.
If referring to a specific physical place eg Il est dans la prison où l'émeute a eu lieu/ He is in the prison where the riot occurred. 
We would use an article in this context in English.