No, unfortunately they do not have rules. You will have to learn which article to use for every noun. "First of all, in Standard Swedish all nouns belong to one of two genders or groups: the en-word group (in which we find approximately 80% of all nouns) or the ett-word group (around 20%).1 It is important to know which group a noun belongs to if you wish to speak good Swedish, since the group belonging affects what endings or special forms of other words to use - but it may be comforting to know that the differences between the groups are not very big, and very seldom would anyone have any difficulty in understanding you just because you have mixed up the groups. English is very simple in this respect, since it treats all the nouns the same way: There is no grammatical difference between ‘a woman’ and ‘a child’, for example.2 In contrast to this very easy-to-learn system Swedish sees ‘woman’ as an en word (‘en kvínna’) but ‘child’ as an ett word (‘ett barn’), and therefore uses two different words for the English ‘a’ to differentiate between them. The rules for telling whether a noun is an en word or an ett word are rather intricate as well as vague, and it would be easier just to memorize each noun together with the article ‘en’ or ‘ett’ to avoid confusion. Words denoting people and animals, though, are, with very few exceptions (among them ‘ett barn’, and ‘ett djur’ - an animal), en words. If you are not sure what form to use, treat the noun as an en word; statistically you would then be right four times out of five. " http://www2.hhs.se/isa/swedish/chap3.htm