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.

Maarten Kamp · 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.