"The man eats an apple."
Translation:Manden spiser et æble.
Like in Spanish and Italian, words have gender (in Danish the genders are "common" (n-words) and "neuter" (t-words)). Unlike Spanish and Italian there's no real way to tell what a word is other than through some endings (which a lot of everyday nouns don't have), and even then there are exceptions. Read here for more information
When, if at all, is the phrase et aeble (the apple) replaced with aeblet (the apple)?
et æble means "an apple", so the difference is the same as English between "an apple" and "the apple"