în - means more or less 'inside' but not only as a location, but also as a an object of time or space. Inside something - inside a church, inside a building, inside as an object of time or space : (was) in highschool, in the past, in the future, in the same class. la - means most of the time 'to' and it simply points to a destination/place - to (city X), to the seaside, to the mountains, (went) to highschool I didn't give any romanian language examples, in order to make it clear (hopefully) through english.