I believe you guys were talking about the American variant of English. British textbooks clearly state that if you are a patient you will use "in hospital" and not "in the hospital", while using "in the hospital" means you are visiting someone there, the same is correct for schools, colleges, churches, prisons, or jails. I was just curious what is the rule if you work there. DO I need "the" with "in"? or should I use "at the" in this case? Again, in British English. Would be nice to hear, if somebody's aware.

"In the hospital" is a common expression for saying someone is a patient in the hospital. If someone tells you, "My dad is in the hospital," they most likely mean their dad has been hospitalized and is being cared for there. It can technically also mean that someone is inside a hospital, but it is rare to hear it used this way.

Shawn is mostly correct. But I think that number 5 usually means that Mark is a patient in the hospital. "Mark is at the hospital" means he is in the building, or on the grounds, like in the parking lot.

You cannot say "at hospital". You have to say "at the hospital" or "at a hospital", depending on what you're saying.

1. I work at the hospital. / I work in a lab at the hospital. (If you are speaking about a particular hospital that the listener is aware of and would recognize from context. For instance, if the only hospital in the area is the one in town.) 3. I work in a lab in a hospital. (If you aren't stressing which hospital you are talking about.) 4. He is at the hospital. (This means he is either visiting a patient or is in the emergency room himself.) 5. Mark is in the hospital now. (This means that your friend is actually inside the building. For instance, if your mother meets you in the hospital's parking lot and asks where Mark is.) 6. I met them in the hospital's parking lot. (We say "in the hospital's parking lot" instead of "at the hospital's parking lot" because the parking lot is a huge area in which we are located. So you are in/inside a boundary here.)