"The cat eats the mouse."
Translation:De kat eet de muis.
Why does paard (horse) follow "het" while Kat (cat) and muis (mouse) follow after "de"?
There are no rules about this, only feeling and enough time will help you understand. Source: I'm native German, and "het" and "de" is very similar to German use of the articles.
When refering to animals, are they all "It" like in english? Or can i refer to them like "Ze is een kat"?
eet is singular eten is plural. Zij eet een appel/She eats an apple. Zij eten een appel/They eat an apple.