"The child eats an egg."
Translation:Το παιδί τρώει ένα αυγό.
The sentence says "an egg," so the Greek for the indefinite article "an" in this case is "ενα". If you included το, you would have to replace it for ενα and that would change the meaning of the sentence. Having both together doesn't make sense in greek or in english. It's like saying "The child eats an the egg."