"I eat this."
Translation:저는 이것을 먹습니다.
을/를 is the object particle. 을 was added to 이것 to make it the object that is being acted upon. I think 이걸 is an informal way of saying 이것을.
They are both fine, I think. 거 and 것 are the same, and they become 걸 and 것을 when adding 을.
I think the particle is missing to make it a object. Because 'this' is an object in the sentence. But I am not sure either why that is. I had 이 too and read the former discussion. It nakes sense that 'this' needs an particle in thia sentence.