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After studying copula verbs `だ` and `です` in japanese grammar books by myself a
couple of months ago, I was under the impresion that `だ` is omittable as it is
implied by the context.

Moving on, I recently began reviewing `na-adj` and `i-adj` with a different
book, and found out plain non-past form of `i-adj` are conjugated **without**
`だ` while plain non-past `na-adj` are conjugated **with** `だ`.

Examples would be:

`i-adj: この時計は大きい`

`na-adj: 彼女が好きだ`

That being said, it `だ` only used with `na-adj` or can it be used with `i-adj`
and ommitted whenever the speakers wants to?
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2015-02-23T04:05:36.847
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question
grammar usage copula
Is だ (plain form of です) omittable?
だ can be dropped after i-adj because the final い in i-adj, or its
conjugations, carries the same meaning as the copula itself.

In na-adj, the final な--used as a link from the adjective to nouns or noun
phrases--is etymologically derived from the copula. ( In 古語, the link is more
easily understood. Changes to the language overtime obfuscated the connection,
barring some regional accents near Nagoya. ) It's use is limited to linking
adjectivals to nominals, however, and therefore is no longer used in the
sentence final form. ( Not to be confused with the sentence final な that
indicates reflection--asking one's self a question internally--as in
俺の説明がくどいな… )

Also, the だ form of the copula is used in casual speech and therefore can be
dropped as a matter of style--just like in casual American English.

vis. "What's that?" "Bread." versus the less curt, "It's bread".
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**だ is not the plain form of です.** They're related, but you can't use だ
everywhere you can use です, so calling one the plain form of the other doesn't
work.

です has two functions:

  1. As a polite copula, similar to だ:

> りんご **だ** → りんご **です** （noun）  
>  きれい **だ** → きれい **です** （na-adj）

  2. As a politeness marker, following i-adjs:

> うつくしい → うつくしい **です** （i-adj)

i-adjs form complete predicates on their own, and there's no need to add a
copula to them:

> ＊うつくしい **だ** ← i-adj+copula, ungrammatical  
>  うつくしい **です** ← i-adj+politeness marker, OK

There's another difference between the two forms. As copulas, だ and です inflect
for tense:

> りんご **だった** → りんご **でした** （noun）  
>  きれい **だった** → きれい **でした** （na-adj）

But as a politeness marker, です does not inflect; the word before it does:

> うつくし **かった** → うつくし **かったです** （i-adj）

It's true that you can generally omit sentence-final だ in informal style. It's
also true that you must omit sentence-final だ before certain particles such as
か and さ. But when you say うつくしい without だ, that **does not mean だ is omitted**
; it was never there in the first place, and it's ungrammatical to add it.
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2015-02-23T07:01:51.083
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answer
Just some observations. Everything is omittable in Japanese, like in many
languages; which adds some spice. Any sentence heavily relies on context, so
in informal speech people can spell just a meaningful half of the sentence.
Still It does not mean the sentence is correct. For me, personally, the
difficulty is finding out where can you omitt だ or である in formal (not
ceremonial) writing, i.e. reports and manuals. Using である too often is
discouraged, despite basic rules; so I try finalizing sentences with other
verbs in るform. When you use です, or で~~ます you usually add it by intention to
honor the opponent. So there is no reason for omitting です; rather replace it
with だ by the rules, or when avoiding text bloating. And yes, both です and だ
probably have (there are theories) similar origins, somewhat related to the
universal verb "be", ある, while です is a reduction of its honorific forms.
Therefore the usage became not exactly equivalent. Another minor theory I've
seen, is that they come from different verbs, "be" and "do". For instance in
Kansay there was a ”や” particle in place of だ, which suspiciously resembles
"やる"(I could not confirm it yet). And Kansai dialect has roots in very old
capital Japanese.
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