You have to learn if it is an en-word or an ett-word. But keep in mind that most substantives are en-words:)

It is hard to explain as you have to learn all articles for the substantives by heart. There are no real rules when it comes to "en, ett" and so on. You can read more about it here: http://www.lardigsvenska.com/2010/08/substantive.html http://www.lardigsvenska.com/2010/08/substantive-del-2-en-ord.html