"The mouse eats the cheese."
Translation:Musen äter osten.
It depends if it's a gender/common or neuter word. It's similar to the masculine and feminine words in Spanish, but in Swedish, the gendered words merged into a single form, and the neuter form was also kept. It's basically a matter of memorization, but 80% or so of nouns are en words.
In this case, both mus and ost are en words.