I think "at hospital" versus "at THE hospital" is a regional difference among English-speaking countries. The same goes for "in hospital" versus "in the hospital".

Hospital is a bit unusual, and has been covered thoroughly below, but other places follow a more general pattern. Places such as: school, college, university, prison, church, etc. He's at school. = He's a pupil. She works at the school. = She's not a pupil. He works hard at school. = pupil She works hard at the school. = not pupil We met at university. = We were studying there. We met at the university. = this is just a place, we aren't necessarily students See you at uni! = we are students there. See you at the university. = we aren't students there I often go to church. = to worship I often go to the church. = just to have some peace and quiet or something He works in prison. = He is a prisoner. He works in the prison. = He's not a prisoner.

When you use "in", it means you are a patient at the hospital, while "at" means the person is just on premises and not a patient.

All of those phrases are possible to say, but they have different meanings and usage. "AT hospital" and "AT THE hospital" usually mean the same thing. They both refer to the physical location of the hospital, and they are used to indicate where someone is located or where something is happening. For example, "I am at the hospital" or "The meeting is at the hospital." However, "AT THE hospital" can also be used to refer to a specific hospital, while "AT hospital" is more general. For example, "I work at the hospital" versus "I work at the hospital on Main Street." "IN hospital" and "IN THE hospital" are also similar but have different connotations. "IN hospital" usually means that someone is receiving medical treatment or care in a hospital, without specifying a particular hospital. For example, "He is currently in hospital recovering from surgery." On the other hand, "IN THE hospital" implies that someone is talking about a specific hospital. For example, "I visited her in the hospital last night." In summary: "AT hospital" and "AT THE hospital" indicate a physical location and can be used interchangeably, although "AT THE hospital" can also refer to a specific hospital. "IN hospital" generally means receiving medical treatment in a hospital without specifying which hospital, while "IN THE hospital" refers to a specific hospital.

You can say "at the hospital" means you are close to the building. If you say you are "in the hospital" you are inside the building