Fri can, as Christian points out, mean independent, unlimited or free. for example, the term "fri bar" literally means "free bar", while the term "free entrance" would be either "gratis inträde" OR "fri entré". Usually, in terms of price, gratis is used.

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.