When do I use 'i' vs 'på'?

The general rule: 'i' for enclosed spaces and countries/cities, 'på' for surfaces, islands, and institutions. So 'i Stockholm' (city) but 'på Gotland' (island). 'I skolan' works because you're thinking of being inside the building, while 'på universitetet' treats it as an institution.

Don't worry, even native speakers sometimes hesitate! The institution vs. building distinction is the key. 'Jag är på sjukhuset' (I'm at the hospital as an institution) vs 'Jag är i sjukhuset' (I'm inside the building).