The most common prepositions of place are an expression of our mental image on the situation. This mental image may have 1, 2 or 3 dimensions (or none).

Where’s Mike?

He’s in his bedroom  (a room has 3D and we are placing him inside)
He’s on the beach  (a beach is a surface, 2D)
He’s at the window  (in this case, the window simply means a place in the room where he is, not inside it or on it, but there, in that place)

Compare:
He’s in the cinema  (he’s inside the cinema, maybe watching a movie)
He’s on the cinema  (he’s on the roof, maybe repairing it)
He’s at the cinema (he’s in that place, not in the park or in the church or at home; like putting a cross on our mental map of the city)

We would also use AT if he is waiting for his girlfriend at the front door, because he is not inside or on the roof. In fact, he is not in the cinema, but we are simply using the cinema as a reference to say where in the city he is.

He’s at the police station  (we are placing him in that part of the city)
He’s in the police station  (we are visualizing him inside the building, maybe because we are thinking about what he is doing there)

If a train passenger is travelling from London to York he will say “I arrived IN York”, because for him, York is a 3D city, with streets and parks and buildings…

But for a passenger travelling from London to Edinburgh, York is only a spot on his route. For him, it is not a 3D city, just a spot, a place on the map, so when the train stops there for a moment he will say “We are stopping AT York”.

SPECIAL CASES

PUBLIC TRANSPORTATION

Of course we must use ON if we are on a bicycle, IN if we travel by car or taxi, but… why ON for the rest? What do the sentences marked in blue have in common? They are all about public transportation, meant for a group of people.

For thousands of years, and until recently, the only way of transportation most people could use was a horse or a donkey, and for them we must use ON, since we are riding on them. When modern transportation was invented, they kept using part of the language used for horses, including the preposition. That’s why we say we drive a car, but a journey is called “a ride”, and when you take the bus you can take a ticket to ride, so it is easy to understand why we are ON a bus if we think of it as the successor of a horse.

But then, why do we say IN for a car or a taxi and not ON? The answer is, again, a psychological motivation. When people get into a car, they feel transformed; most people get more aggressive and self-important, almost invulnerable. Why is that? Because we feel extremely safe inside a car. It is the same feeling a medieval knight would feel inside his armour. So the intense feeling of being INSIDE a protective shell is too strong to use ON. The idea of horse is here replaced by the stronger idea of armour. But inside a bus you don’t get that feeling, there’s no armour there to protect you from other people, if you insult them, they can hurt you, so there is no protection for you.

IN BED

If you are reading a book or listening to music on your bed, we use ON, because you are on top of a surface. But if you are inside the bed (under the covers), we say IN THE BED. Nevertheless, the bed is a special case, because when you are inside we may or may not use the article THE according to the following distinction:

A bed was created to sleep or to stay through your illness, but it was not designed to read or watch television. If you are inside the bed doing what you’re supposed to do there (sleep, etc.), then we say IN BED, but if you are doing something else (playing, reading, etc.), we usually say IN THE BED. This difference is important for some other situations we will see later.

AT HOME

He’s at home / He’s in his house
He’s at work / He’s in his office

“Home” and “Work” always use AT. Why? Because they are not places, they are concepts. “Home” is where you live (a house, a palace, under a bridge, in a cardboard box). Since it is a concept, we are not talking about a thing, a building, but about an action (to live, to work), and an action has no dimensions (0D), so we must use AT. Or if we think of it as a place in the city map, then we have 1D, and again we must use AT. But a House is a building (3D), so we can use IN.

The same goes for “Work”, it is not a place, but a concept. You can work in an office or the city council, or a field, or on a place or in the sea, and that is where your work is. But if we say “office”, for example, then we are talking about a room with 3D, so we use IN.

So this distinction shows that when we are thinking of a 3D place we use IN, but when we are thinking of an action (0D) we use AT.


PUBLIC BUILDINGS

The same distinction we saw about place (IN) and activity (AT) applies for public buildings, but here we must also remember what we said about the bed.

He’s at school        (a teacher, a student)
He’s in the school         (a student's father, a painter)

A school was built to teach (teachers) and to learn (students). A school is a building (3D) but learning is an activity (0D), and you can also have a class under a tree. So this is the same situation as “home” or “work”. On the other hand, we also apply the distinction we saw for bed, considering the original purpose for that building. So now we have a difference in the preposition and also in the use of the article.

If Tim is a student and goes to school to have a class, then we say that he’s AT school; but if he goes to the school by night to paint some graffiti on the walls of his classroom, then we say that he’s IN the school, because he is not using the building for its original purpose (remember the bed), he didn’t go to the school as a student, but as a vandal, and schools are not built for vandals, so we use IN and the article.

If a teacher is there teaching, he’s at work, so  we say he is AT school. But if the cleaning lady goes there to work, she’s at work, but it is not the same situation. A school is built so that teachers can teach, and they also need cleaning, of course, but a school is not built so that the cleaning lady has a place to clean. She is not there for the primary purpose (teaching or learning), so we say that the teacher is at school (only if he is working), the student is at school (only if he is learning), but the cleaning lady is in the school. Also, if a father goes to the school to talk to his son’s teacher, or if a plumber goes to the school to repair a broken pipe, we say that they are IN THE school. Another example:

He’s at the disco (he’s gone there to dance, or he works there as a waiter or DJ)

He’s in the disco (he’s in there painting the walls, or interviewing people, etc.)

He’s at church
He’s in the church

A church is built so that people can go there to pray or for mass, and so that priests can do their job there. So if a worshipper or a priest is there, we say that he is AT church, but if a woman goes there to take some photographs of the beautiful altarpiece, then she is IN THE church. You can take pictures in a church, but a church is never built so that people can take pictures.

He’s at the hospital          (a doctor, a patient)
He’s in the hospital          (a visitor, a painter)
He’s in hospital          (a patient in bed)

There are some public buildings where people not simply go to spend some time, but they can also go there to stay, that is, to sleep in a bed. In that case we don’t have two, but three different situations. You can go there to do what you are supposed to do, or you can go there for another reason, or you can go there to do what you are supposed to do and sleep there.

We say that the doctor is AT THE hospital when he is there at work and we also say that the patient is AT THE hospital when he goes there to see the doctor. We say that the painter is IN THE hospital when he goes there to paint the walls. But we say that the patient is IN HOSPITAL if he stays in hospital for a few days, sleeping in bed. For the same reason that we say “he is in bed” when he is sleeping, we would say “he is in hospital” when he stays there and sleeps there. So this is what you can get:

He’s at the hospital  (the doctor, the patient)
He’s in the hospital  (the painter)
He’s in hospital  (the patient, in bed)

And this is another example of a similar situation:

He’s at the prison  (the guard)

He’s in the prison  (the visitor)
He’s in prison  (the criminal