I think something has to be a bit more boxlike/cylindrical etc. until you can say it. Or things like: in the middle of smthg. I mean, there are many more incidents, but for objects they should be more closed.
It can be translated as "in", but that would be a completely different context. "Han sade det på svenska." "He said it in Swedish." Prepositions rarely have one single direct translation, they're often context-dependent.
I wrote "the fork is for the plate" as I thought there is someone giving it to someone else to put it there when one is missing. Could you say it like this? Or would it be "för tallriken"? I still confuse för and på.