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ばかり, when used to add the connotation of having _just_ done something, usually
(or nearly always, in the examples I've seen), takes a copula after it. Either
だ or です.

An example:

> 彼は帰ったばかりだ。

My question is: does ばかり, grammar-wise, _have to_ take a copula?

Asking because だ/です is only necessary for noun sentences, while here I see no
noun. ばかり does not look or act like a noun and is usually listed as an
(adverbial) particle (副助詞). Is it just a particularity of language, and some
words like ばかり despite not being nouns, just require だ? Is the だ grammatically
unnecessary but usually used because it adds emphasis? Is ばかり somehow a noun?
Or perhaps does ばかり turn the whole phrase before it _into_ a noun?
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2021-04-22T23:33:35.887
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question
grammar copula
Why is ばかり used with だ / です
> My question is: does ばかり, grammar-wise, _have to_ take a copula?

In this position in a sentence, yes, the copula is required.

Historically, ～ばかり derives from a noun. I suspect that informs the grammar
point that requires the copula after this. Looking at some quotes from older
texts, where the preceding word is an inflecting word, the inflection used is
the 連体形【れんたいけい】 or adnominal form, which is used to modify a following noun.
(Note: The 連体形【れんたいけい】 and 終止形【しゅうしけい】 or terminal form have fused in modern
Japanese, but there were still distinct endings for these in Classical
Japanese and older forms of the language.)

Likewise, だけ also derives from a noun, and similar grammar applies --
彼は帰っただけ[だ]{●}, where that copula is again needed.

See also [this answer
post](https://japanese.stackexchange.com/questions/47505/what-
does-%E8%A8%B1%E3%82%8A-bakari-mean-and-what-happens-when-you-combine-it-with-
ka-ni-an#answer-48845) about the derivation of ～ばかり.

> Is ばかり somehow a noun? Or perhaps does ばかり turn the whole phrase before it
> into a noun?

Yes, and yes.

The first "yes" is answered further above: ばかり is derivationally a noun, and
grammatically still behaves as one in some ways.

The second "yes" is due to the way that Japanese allows for various words and
phrases to modify the word directly afterwards, without requiring any
coordinating conjunction similar to English _that_ : "the car _that_ goes" can
be rendered in Japanese as simply [行く]{いく}[車]{くるま} (literally "the goes car").

From this, let's take another look at your sample sentence.

  * 彼は帰ったばかりだ。 
    * The topic is clearly 彼は. That's what we're talking about. Topics are the outermost context of the sentence. In some (many?) cases, as in this one, the topic is also the subject, even though there's no explicitly stated subject marked with the particle が.
    * Then we have the past tense / completed verb 帰った.
    * But then we immediately have that ばかり. The 帰った essentially modifies the ばかり, telling us more about what kind of ばかり that is.
    * And since ばかり is (functionally) a noun, we then need that だ on the end to close the predicate.

* * *

Please comment if this does not address your question, and I can edit to
update.
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