What does "da" mean in "tabeta bakari da"?


I understand (correct me if I'm wrong) that _da_ is a short casual form of
_desu_. But what does it mean in this phrase? or why it is used?

> _Tabeta bakari da._




>   1. [食]{た}べたばかりだ。(Tabeta bakari da)  
>  I have just eaten.
>
>   2. 食べたばかりなの？(Tabeta bakari nano?)  
>  Have you just eaten?
>
>   3. 食べたばかりではない。(Tabeta bakari dewa nai)  
>  I have not just eaten.
>
>

There is a writing style named 論文調 that is for an essay in Japanese, and the
end of sentence is 'da/dearu' meaning the assertion. It seems that any
examples of the language textbooks use this style usually. That is a
mechanical style at the opposite end of the polite style of expressions such a
honorific language, and it is a kind of literary style.




ばかり【bakari】 is a 副助詞 (adverbial particle), which is derived from the 連用形
(-masu stem) of the verb はかる. But the particle (and 連用形 in general) behaves
much like a noun. (Join to other noun-like words with の, make into a predicate
by adding だ, etc.)

Now you essentially have a noun phrase 食べたばかり. To make a sentence out of this,
you have to add だ・です (or だった・でした in the past tense).

This is the same as in

> 本だ。  
>  _hon da._  
>  [This] is [a] book.

