He works at/in the sales department.
in school, at school
Which proposition is correct in each sentence? They are confusing. Thanks a lot.
Either preposition could work in all of your phrases; it depends on the context and the perception of the speaker and the location of the facility. At refers to a point reference (He's not home; he's at the shop) and in refers to a 3-dimensional situation (He's not in the shop; he's in the storeroom).
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Chariotif the above phrases do not contain "the", there is less sense on the physical side. Is this perception correct?Yes, that is the general idea.
in the school - under the roof of the school building (said of a person OR thing)
at the school - in or near the school building (said of a person OR thing)
in school - being educated in or at a school; occupied with being educated (said of a person)
at school - located at a school; attending a class in a school (said of a person)
You can say that there is an auditorium in the school, but the auditorium is not in school or at school.
You can say that there is a playground at the school, but the playground is not in school or at school.
Auditoriums and playgrounds cannot be educated or attend classes.
In general, at X means participating in the activities associated with X. at school, at work, at court, at prayer, at band practice, at football practice.
They could, or
"in school"= a student
"at school" = not at home
It depends on context.
The meaning of 'enrolled in' is expressed by  in BrE and by  in AmE; the meaning 'at the place, not at home' is expressed by  or  in BrE, and by  in AmE; the meaning of 'within the building' is expressed by  in both BrE and AmE.
Quoted from CGEL, Section 9.16, Longman.
Not quite accurate, Sitifan, at least re at the place in AmE-- both are at and in are fine. See CJ's comment on context.
Mister Micawber.What did you mean by [at least re at the place in AmE]?
I haven't mentioned [at the place] in my reply, have I?
the meaning 'at the place, not at home' is expressed by  or  in BrE, and by  in AmE.