Re: Swedish: varför byter de skepnaden
If the text was OCR:ed it could be that it said skepnader originally.
Or if typed in by a non-native speaker, it would be easy to see an n instead of an r.
I'd say (also not being an expert at grammar) that if you speak of separate types of particles having separate types of shapes, you would us this form.
To clarify: In the example of bostad, if one family moves, even multiple times, you'd use bostad, but if you were talking about a multitude of families you would (or at least could) use bostäder.
...men på den tiden fanns inte Wikipedia, så man fick klara sig med att förstå själv.