Which one is correct?:
I was at the cinema yesterday. / I was in the cinema yesterday.
I was at a concert yesterday. / I was in a concert yesterday.
I was at the theatre yesterday. / I was in the theatre yesterday.
Why is sometimes "at" correct in different contexts and "in" in others? Can "at" mean "near" and "in"? If so, how could you guess that the speaker when he uses "at" means "in" or "near"? I know that "in" means inside (correct me if I'm wrong) but it's very confusing when it's "at" but not "in". I don't know if the speakers means inside or near. How is it possible to guess?
Is "at" more preferable in sentences like: "I was....". Is it some kind of a safer way not to make a mistake when you are tempted to use "in"? How do Americans and Britons see this?