I am not an expert, just a student who is currently finishing her Swedish studies. At school they told us that there was no rule for the en and ett words. You just have to learn them the way they are and that is all. Sometimes you can guess in which case use en and in which use ett, by how they sound, but that is not the best idea. Best of lucks! So, sit, relax and enjoy the ride! Swedish language is fun to learn!

I think that maybe that's the most difficult thing about the Swedish grammar, because there are no rule, you just have to learn it. But how ever you say it people will understand since the meaning is the same. Good luck :)

I just checked your comment. Thank you for your explanation!

Think of it this way: why is it aN apple instead of A apple? It's the same thing with Swedish. there are no proper rules for this really, some words are just prefixed with either ett or en; both of them have the same meaning. Ett hiss would raise a bnch of eyebrows here in sweden because the proper term would be "en hiss", which in english would translate to "an elevator". En apelsin translates to "an orange". With this in mind, if it's an in english there's a big chance it's "en" in swedish. However, the term "teacher" for example is tricky because if you were to say "a teacher" you would go "en lärare", even though in english it's not an. the swedish rules for this adheres to the same set of rules that german does with feminin/masculin/neutral, ie; you just have to have a feel for it hehe. Don't worry, you'll get it pretty soon. It will just feel and sound odd once it's there and you will know what to use where.