I'd appreciate a clarification on when it would be best to complete this sentence with "at" and when with "in".
closed as too localized by J.R., Carlo_R., Brian Hooper, tchrist, kiamlaluno Feb 25 '13 at 13:57
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Whether you should say “Mr. Dill works at a big library” or “Mr. Dill works in a big library” depends on what the facts are and your intended emphasis. If Mr. Dill is a librarian who works inside the library, either form can be used with little difference in meaning and no difference in grammaticality. If Mr. Dill is a groundskeeper who works for the library but outside it, I'd say at, not in, because it is misleading to say that he works in the library.
In the example sentence, you can use any of the prepositions at, in, or for, with the same meanings for most employees. If you want to emphasize who Mr. Dill works for, use for or at, not in. If you want to emphasize where Mr. Dill works, use in, inside, or outside.
One works at a place, not in it. That’s why we say that someone is at work, not that they are in work. One always works at a business. Oh sure, you might say that someone is “in” real estate, but that’s different.
This is true even if you name a specific building, like saying that someone works at the the Sears Tower or at the downtown library or at the customs house. These are all always at, never in.
Only if you were talking about a specific physical building and specifically contrasting working outside from working inside might you ever any occasion to use in, and even then you would need more supporting contextual orientation.
Really, the place where you use in is for cities, states, or countries. He works in Chicago, not at Chicago. But it would then be at the Sears Tower, and for this-or-that business there. But don’t get carried away with the for; people can work “at” a business, too.
Similarly, someone can work at CERN, but in Geneva. Even if you knew the particular building, it would never be in, only at.