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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?

Christopher Francisco
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3 Answers3

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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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    As a matter of fact, いいです and the like are a modern construction. In the old system, which would have had ようございます（よかったです＞ようございました), it is very easy to analyse です as the polite form of だ. It's probably better to view there as being two です: one is the polite form of だ; the other is a politeness marker for 形容詞. – Angelos Jul 12 '15 at 21:57
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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".

StudentsTea
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  • So, for i-adj, だ can or must be dropped? for na-adj, must it be used? for any other case, can it be dropped? – Christopher Francisco Feb 23 '15 at 06:47
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    For i-adj, the だ form of the copula must be dropped. More properly, it must never be there in the first place, because the い in i-adj is a copula. So, putting both would be like saying, "My pizza is is cold." – StudentsTea Feb 23 '15 at 07:03
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    Other forms of the coupla are okay after i-adj ever, because they indicate tone ( politeness, formality, etc. ). So, 寒いです is fine, because the い serves as a copula modifying the adjectival, and the です indicates tone for the entire phrase--in this case, neither polite nor informal. – StudentsTea Feb 23 '15 at 07:06
  • When to drop the copula in other cases is a matter of style. For example, when it's appropriate to say, "Bread," versus when it's appropriate to say, "It's bread." For that, I would recommend simply consuming as much Japanese media as possible. ( Emphatically, not just manga and anime. ) For example, I'm an American from a blue collar city, so I liked dropping the copula a lot. People in blue collar parts of Tokyo were easier for me to talk to; they tend to be curt, compared to their Yamanote counterparts on the upper west side. My grandmother, on the other hand, is very upperclass; ... – StudentsTea Feb 23 '15 at 07:12
  • ( continued ) My grandmother came from an old, elite Tokyo family. So, to this day she readily jumps into super formal Japanese ( vis. そうでございますか, で being the gerund of the copula, ございます being a formal word indicating existence. ) Next to her, dropping the copula makes me feel very, very curt. – StudentsTea Feb 23 '15 at 07:15
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    If you'd like a good book on this stuff, I recommend getting all three volumes of "Japanese: the spoken language" by Mari Noda. They do an incredible job of crystallizing the linguistics of the copula--and all other fundamental, spoken Japanese. – StudentsTea Feb 23 '15 at 07:19
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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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