For school, university, hospital, prison, church, etc., it depends what you're doing there. You don't use an article if you're using the building for it's main purpose. He is in prison at the moment. - He's a prisoner. He's in the prison at the moment. He's not a prisoner - at least not a permanent one in that prison. He works in prison. - he is a prisoner with a job in the prison. He works in the prison. - He is a guard or something. I met him at university. - We were students there. I met him at the university. - We were at the university for some other reason. I sometimes got to church. - to worship I sometimes go to the church to study the architecture. For television, without the article it is the abstract noun and with it it is the device. Sometimes they can mean the same. I saw it on TV. - Not at the cinema, etc. I saw it on the TV. - A device Sometimes you have to use one or the other though. He works in TV. Someone is coming to fix the TV later. Interestingly, it's different for most other media as we usually have to use an article when saying we used it. I heard it on the radio. - not on radio I saw it on the internet. I read it in the paper.