Can native English speakers explain this?

Non-native speakers often get confused with using proper prepositions "in" and "at".

Which position should be used for the following two sentences?

A) I'm (in/at) the library right now.
B) She was sitting (in/at) the bar drinking Margarita.
C) He's working out (in/at) the gym.

If either position can be used for the sentences above, could you briefly explain when "in" is generally preferred and when "at" is preferred?

In all these cases, both "in" and "at" are acceptable, but "at" would be used rather than "in" in most cases unless one wants to specifically emphasize that the person in question is *inside* that in question.

To continue with what Travis was saying, note that in example B 'bar' has slightly different meanings. 'She was sitting in the bar' means that she was somewhere inside the building that's called a 'bar' (because a long counter at which drinks are served -- also called a 'bar' -- is located there). 'She was sitting at the bar' means that she was sitting next to the long counter itself rather than at a table or somewhere else.

'At' often has a slightly broader meaning than 'in'. Sometimes it means inside, but sometimes it means outside but nearby. In the first example 'I'm in the library' means that I'm inside the library -- inside the building or room that holds the books. If you're talking on the telephone, you're talking from inside that physical structure. If you call from a pay phone right outside the library, you wouldn't say, 'I'm in the library'. You'd say 'I'm at the library'.