"A glass of water"
Translation:Ett glas vatten
The words differ, some substantives have en, some have ett. I don't think there's a definitive rule for that, at least none was ever taught for us. Ett slott, ett ägg, ett äpple, en kniv, en fågel.
That's confusing! I thought "ett" is only used if the noun is fem/masc.. In German i. e. "the glas" is neuter. ("das Glas")