Is there a good reason to choose one form of the passive participle over the other?
The difference is subtle. blev stoppade shows it as a point in time whereas stoppades shows it more like a process. Since this is the kind of event that typically happens as a point in time (at one point you haven't been stopped yet, at another point you have been stopped), blev stoppade is usually best.
Why is it stoppade here, but "jag blev stoppad av polisen" in a different question?
Stoppade is the plural of stoppad since it’s ’we’ who are doing it (many people) and not e.g. ’I’ (one person).
Ah, I didn't think the passive would take agreement. Is that why it can also appear as "stoppat", if it were referring to, say, tåget?
Yes, the perfect participle is like an adjective and also needs to agree. For weak verbs it’s:
- stoppad - stoppat - stoppade (stoppa) - note the plural -e (not -a!)
- läst - läst - lästa (läsa)
For strong verbs it’s:
- sjungen - sjunget - sjungna (sjunga)
- skriven - skrivet - skrivna (skriva)
I think this point should be included in the “Tips and Notes.” Very helpful, and clarifi3s all of the different forms in the sentences.
I think it should be accepted, so I'll add it. We do have the word säkerhetsvakt too in Swedish, but these things don't really correspond 1=1 a lot of the time.
It's not very idiomatic English to "become" stopped; rather, you "are" stopped.