"The fox does not eat an insect"
Translation:Η αλεπού δεν τρώει ένα έντομο
An attempt at giving an explanation:
You can say Η αλεπού δεν τρώει έντομο with the implication of a follow-up clarification that τρώει (κάτι άλλο) or, even in absence of a clarification, you intend to imply that something else is going on (the fox is drinking water/sleeping, eating cake). Otherwise it is natural to include ένα just as you do in English.
Η αλεπού δεν τρώει ένα έντομο = The fox does not eat an insect OR The fox does not eat one insect, it eats two / eats a whole bag of insects, can't you see? :) So the version with ένα in fact also contains the one above as well. It's all in intonation, context etc. This is a natural structure, even if the sentence is a bit unnatural in my opinion. What does "The fox does not eat an insect" mean? That foxes don't eat insects? That this specific fox does not eat an insect for breakfast? "The fox is not eating an insect" would make much more sense to me but of course I may be reading this wrong.
Η αλεπού δεν τρώει έντομα = The fox does not eat insects
I'm afraid the reason why this has not been answered in a while is because it's very difficult to explain how including/omitting the article works and what all these variations may imply. I understand that article usage creates a lot of issues for learners. At the same time, I want to point out that sometimes it's for best for us to just let people absorb the material in Greek without trying to explain idiomatic usage or anything that just is and does not stem from a clear rule. :)