Why does 他吃了饭 sound incomplete? As I study usage of different aspects markers in Chinese, I started to realize that my problems with those markers come from simple fact that: "I don't see any use in them." Reason for this is: Whenever I see a bare verb phrase, I translate it as simple present time. But this is far from truth. 

So, let me ask a question:

"Why does 他吃了饭 sound incomplete?"
This kind of sentence is being used in many theoretical talks and papers on Chinese linguistics

My answers:
The difference between 他吃饭 and 他吃了饭 lies only in that 了 renders the situation as factual rather then irreal (habitual, desirable, wishing, issuing a command etc.) 

Explanation:

他吃饭
can be translated (in proper context) as:


An order. "He (must) eat (now)!" (screaming).
A habit. He eats (always).
A desire. He (wants/thinking about) to eat.


But once you add 了 to a verb(as in 他吃了饭),  you thus delete all the mentioned above as a possible translations. Now the situation is "factual \actual". It is not about habit, orders, desires and other irrealis modes. But rendering situation factual is not enough, you still have a big list of possible translations (in proper context), for example:


A fact. He ate.
A fact. He eats... and then... (not as a habit, but actually eats as in some narrative)
A fact. He just initiated a process of eating.
Future. He will have eaten (by the time ...) 
any suggestions?


that is why in 他吃了饭 you need further specification! The verb 吃了 is not rendered to ANY time (just rendered to reality, making it a fact or eventuality)!!!

P.S. Please, refrain from "perfective aspect marker 了" viewpoint if it is possible. Thank you!

P.s.s. Please, provide your possible translations and contexts for 他吃了饭 (this is very important for not natives! We need to see the point in adding aspects to naked verbs!!!)
I can't for the life of me understand why switching an associative verb to the front forms a question in English. Am I not stupid? 

The fact is, in a lot of cases, languages don't follow any kind of logic or reasoning. 

If you say 他吃饭了, it's a complete sentence. If you say 他吃了饭, a listener would expect you to tell him what happened next, like 他吃了饭，拍拍肚子就走了。Now that it's complete, the listener can rest in peace.

So in this case, 他吃了饭 means "After he ate" -- Apparently an incomplete sentence. 

But why does switching the position of 了 change its meaning? No one has the slightest idea.

There are situations where "verb + 了 + object" makes a complete sentence. Like when answering "他吃了什么?" with "他吃了一只火鸡."