Is だ (plain form of です) omittable?

Question

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?

Answer 1

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.

Answer 2

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".

Answer 3

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.