One of the most challenging parts of the English language to learn are the prepositions. Native speakers tend not to think about these or analyze them a lot. Yet for people learning English, especially in the beginning stages, they can be confusing.
In: we use for specific locations. Example: I am in my kitchen.
On: Objects, and people, “on” something: My keys are on the table. (Your keys can’t be in the table, but they can be in the drawer.)
At: general location: I am at the hospital. Great, but where exactly?
My most common example I use with beginning learners:
I am at the hospital in the emergency room on the first floor.