Nothing to fix - Turkish doesn't use indefinite plurals on objects - see https://www.duolingo.com/comment/7736911 .
So "The men eat apples" is "Erkekler (plural, because it's subject) elma (singular, because it's an indefinite plural object) yer".
"The men eat the apples" would be "Erkekler elmaları yer", with plural (and accusative case ending).
Apples are countable; we usually talk about one apple or two apples or many apples.
If people like to eat apples generally, we would usually use the plural (without article) rather than speaking about "eating apple" the way we would with a mass noun such as "eating soup" or "eating mashed potato".
You are wrong. The word "man" generally refers to the word "adam". The word "male" refers to the word "erkek." The word "boy" refers to the word "oğlan". We sometimes say "erkek" for the word "man" depending on the context. But we don't say "boy" for the word "erkek". However, we can say "erkek çocuk" for the word "boy", which actually refers to the word "oğlan" again.