in expresses, inside a space (i.e., in the office).Originally Posted by K.Margaret
at expresses, a location (i.e., at the office).
"I'm at work" (i.e., at a location), but not, "I'm in work" (inside a space). 'work' is an abstract noun. It doesn't have a space. home is another e.g., I am at home, but not I am in home.
at could mean inside or outside
I'm at the bank/store. (location; you could be either inside or outside the bank/store)
in means inside
I'm in the bank/store. (inside the location; you are definitely not outside the bank/store)