Learn Icelandic: Ég skræli eplið áður en barnið borðar það. - I peel the apple before the child eats it.

Áður on its own is an adverb that means "previously" or "beforehand" (like in "I have seen this before"). When you want to link two actions and say "before [someone does something]", you must use the full subordinating conjunction áður en.

Why does the sentence end with það instead of another word for "it"?

In Icelandic, pronouns must match the gender of the noun they replace. The word það refers back to eplið (the apple). Because epli is a neuter noun, you must use the neuter pronoun það.

Both eplið and barnið end in -ið. Is this the same grammar rule at work?

Yes! Both epli (apple) and barn (child) are neuter nouns. The -ið suffix is the definite article ("the") added to the end of the word.

What case are eplið and barnið in here, and why didn't their endings change?

Barnið is the subject of the second half of the sentence (nominative case), while eplið is the direct object of the verb skræla (accusative case). Neuter nouns are very friendly for learners: their nominative and accusative forms are always exactly the same, which is why neither ending looks different here.