Can we also say "Jag stoppades av polisen." here or is there a rule that we must decide to use either "s-passive" or "perifrastic passive" according to something?
Like 'was' vs 'became', the difference between 'state' and 'change of state'. Or like 'things were like this for me' versus 'this happened to me'.
However this doesn't mean there's a 1-1 match between the languages.
Ok i seem to get the difference now. So it would be wrong to say Jag var stoppad av polisen?
Yes, that sounds wrong because it sort of contradicts itself with this version of stoppa, which occurs at a point in time and does not result in an ongoing state – stoppa when the police stops you refers to the act of pulling you over. It may lead to your becoming other things as an ongoing state, like häktad 'detained' or 'remanded'. So you can bli stoppad av polisen and then you may vara häktad or arresterad 'arrested' for some time afterwards.
Does "stoppad" also mean "arrested", or is there a different word for that?
Note you cannot actually be "arresterad" in Sweden! The equivalent is "gripen" :P
You mean in legal terms. Yes, that's true. :) However, we do use the word colloquially quite a lot.
I just thought of this very unlikely event: imagine a robot that was stopped by the police. If that robot were to tell you about what happened, would it say 'Jag blev stoppat av polisen'? (stoppat istället för stoppad). I know it's a stretch, but looking at the future, this could well be a sentence that could start making sense... :)
Consciousness is very much a common-gender thing. I can't imagine a robot not using stoppad, it sounds completely - ironically - unnatural. :)