What is the difference between "the boy eats eggs" and "the boy eats egg"? Or is it the same? In one, egg is a plural, in the other it is a substance. Is it just treated as a plural both ways?
The singular would require indefinite article, that's how you can tell.
The boy eats an egg = Pojken äter ett ägg
The boy eats eggs = Pojken äter ägg
It's the same word for plural and singular (ägg), unless you put an article on there in which case it's ägget for "the egg" and äggen for "the eggs".