Most words are en in Swedish, about 80% i think. It's the other way round: one of the few rules we can give you is that most living things are en. Not all of them though, there are many exceptions: ett barn (a child), ett lejon (a lion) and others.
What happens is that "en" works with common gender nouns (the merge of masculine and femenine genders in just one article), and "ett" works with neuter nouns -like "barn", which is also neutral in Norwegian and Icelandic.
The construction without the article is used for professions, political persuasions and similar things, not just for defining things. I wrote something about this here: https://www.duolingo.com/comment/5852643