at and in (hospital/ bank/hotel/college)

I have noticed in my readings that both prepositions (at/in) occur with places, and sometimes the same place. There are differences in meaning, however.

But what are these differences?

Let me try here.

"at" desginates a place in general. So, when we say: He is AT the bank. We mean he went to the bank. So maybe he is AT the door right now OUTSIDE the bank.

But if we say: (He is IN the bank) we mean that he is INside the bank. There is NO probablity that he could be somewhere around the bank...he is INNNNNN the bank.

I hope I am correct here....another difference between the usage of both "at" and "in" here is probably related to time and location.

When I was ___ (at/in) college, I used to read 50 pages a day.

Here, we will choose IN because we are talking about time ...not location...he may have been reading AT home 50 pages a day.

I hope I am correct but also hope you could elaborate more on the differences between them.

I remember reading:

- in the bank

- at the bank

- in school

- at school

- in the hotel

- at the hotel

and just to confuse some people... I also read (arrive in a country) and (arrive at a place).

Prepositions are probably the hardest part of English to master.

Mucho gracias!!!