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In this sentence:

> _Neko da kara desu._  
>  Because it's a cat.

Why are the "da" and "desu" used at the same time? Logically, it should've
been:

> _Neko desu kara desu._

Or in a casual conversation if we don't drop the last "da":

> _Neko da kara da._

Right?
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2016-05-01T10:56:24.520
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grammar word-choice verbs politeness copula
Noun + da kara desu -- why are da and desu mixed? why not both "da" or both "desu"?
This is one of those funny nuance things. It's got little to do with grammar
and more to do with how it **feels** to say that. If I wanted to sound
condescending I'd say it like that, though I guess to get that effect tone of
voice is also important.

You would never in a million years say desukara desu. That just doesn't work.
9 times out of 10 you would just say neko dakara. Nothing more.
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2016-05-01T13:52:52.717
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Basically, the casual form is ～だからだ and its polite form is ～だからです. e.g.

> ネコだからだ(よ)。  
>  人間だからです。  
>  坊やだからさ。

The から is a 接続助詞(conjunctive particle), definition #1 in
[デジタル大辞泉](http://dictionary.goo.ne.jp/jn/45491/meaning/m0u/):

> 2⃣ [接助]活用語の終止形に付く。 １ 理由・原因を表す。「もう遅い **から** 帰ろう」 (attached to the predicative
> form of 活用語. 1. indicates a reason or cause.)

The から needs to be attached to the predicative form such as ～だ or ～する, so here
you get ねこだから (not *ねこから), then you use a copula だ or です to end a sentence
(though you can just say ～だから。 in casual conversation), so you say ねこだからだ or
ねこだからです.

You don't say *ねこですからです because you wouldn't need more than one polite marker
in one clause, I think...
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2016-05-01T14:43:41.000
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