Tricky Prepositions

"In hospital" or "at hospital"? "In school" or "at school"? With an article or without?  Prepositions and articles make many learners rack their brains and think two, three times or more. If you are one of them, and chances are you are (which is good because it means you care about what you say and how you say it), then read this article. We'll try to clarify a thing or two.