In Swedish there are two ways to say "one" - "ett" and "en". Ex. one table = "ett bord" and one cat = "en katt". There is no rule for when to use either. And yes, this makes it extremely difficult for foreigners.
I do agree that it's difficult to learn though as there are no rules for determining a noun's gender or lack thereof. I certainly don't miss memorising the genders of nouns in primary school, not to mention all the exceptions.