This is what I have in Scott Mellor's "Beginning Swedish" "There are five basic endings for plurals in Swedish, two for neuter (ett) nouns and three for common (en) nouns. The rules for (ett) nouns are relatively easy: if the noun ends in a consonant there is no ending; if it ends in a vowel the ending is -n. Plurals for common nouns are slightly more complicated: if the noun ends in a -a the plural is -or. There are two more plural ending for nouns, the rules for which are too complicated to be of any value, therefore it is best to memorize them. en flicka /flickor en pojke /pojkar en cigarrett /cigarretter ett äpple /äpplen ett barn /barn"

I don't think there's a rule. So you will have to learn them all individually!

You can find some good information about it here: http://www.thelocal.se/blogs/theswedishteacher/2010/07/28/plural-endings/ http://www2.hhs.se/isa/swedish/chap4.htm

Good question. :)