Why it's say "He works at the post office" and not "He works IN the post office"?
In senteces like "He works IN a pharmacy" is used IN and not AT, but when you say "He works AT the post office" is used AT.
When I should use AT and IN?

Question

Answer

You can say: "He works AT a pharmacy."
at a place = within its boundaries. This can be inside a building or in the space surrounding it.
in a place = strictly inside.
That is why you say "in an office". But sometimes you are "at the office", meaning you are not strictly inside the office, you are just at work. (You could be out for lunch)