"It is eating an apple."
Translation:Den spiser et eple.
It depends one what "it" is referring to, and the gender of that thing.
"En hund spiser. Den spiser et eple." (dog is masculine)
"Ei katt spiser. Den spiser et eple." (katt is feminine)
"Et ekorn spiser. Det spiser et eple." (squirrel is neuter)
"Ekornene spiser. De spiser et eple." (plural)
You have to memorise each new noun you encounter in full (indefinite article + noun itself) because there aren't any rules you can follow to determine which article goes with which noun; also, it's totally irrespective of the biological gender.
Good news is that you only have to deal with two genders because you can use en for nouns of feminine gender as well.
The use of "den" here implies that a m/f subject has already been introduced prior to this sentence.
We don't know what it is, as we don't have more than a single sentence to go on, but we can imagine that it is a dog, for instance. In regular conversation, you would have the context needed. As it stands, the English sentence can be translated using either "den" or "det", as we don't know what sort of noun the pronoun "it" is referring to. It's not used as a dummy/formal subject in this sentence.