어 is an ending suffice used for present tense. 밥을 먹어 means 1. I'm eating meal. (declarative sentence) 2. Eat meal. (imperative sentence) 3. Are you eating meal? (interrogative sentence) V-는 거 is a Noun Form transformed from the verb to be used as an object or an subject in the sentence. 보다 to see (a verb) → 보는 거 seeing (a noun) 영화 보는 거(것을) 좋아해요. I like seeing movies. 밥을 먹는 거야 means "be eating meal." It is not a full sentence. It depends on the context. ex) 배고프니까 밥을 먹는 거야. For I'm hungry, I'm eating meal.