Is だ (plain form of です) omittable?


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?




**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.




**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.

