です / だ (desu / da) — The Declarative Copula
The Japanese equivalent of 'is/am/are' — the copula. It links the subject to a noun or na-adjective to form a complete predicate. です is polite; だ is plain/casual.
Structure
[Noun/na-Adj] + です (polite) | [Noun/na-Adj] + だ (plain)
Examples
これは辞書です。
Kore wa jisho desu.
This is a dictionary.
これ (this) = 辞書 (dictionary) + です
彼は親切な人だ。
Kare wa shinsetsu na hito da.
He is a kind person.
だ = plain/casual copula declaration
学生じゃありません。
Gakusei ja arimasen.
I am not a student.
じゃありません = polite negative of です
• Key Distinction: The copula does NOT mark physical existence (that is the job of いる/ある). It asserts identity or a property (A is B). • i-Adjective Rule: With true い-adjectives, です is only a politeness marker (寒いです). Because い-adjectives are already complete predicates, adding だ to them is ungrammatical (寒いだ is wrong; casual is just 寒い).
Commonly confused with
The JLPT rarely asks whether you recognise a pattern — it asks which of several similar patterns fits. These are the ones です is tested against.
vs いる / ある (existence)
です/だ equates two things ('A is B', identity); いる/ある state that something physically exists or is present ('there is / I have'). '猫です' = it is a cat; '猫がいます' = there is a cat.
vs い-adjective predicate
な-adjectives and nouns need だ/です to become a predicate (元気だ); い-adjectives are self-standing predicates and take です only for politeness, never だ (寒いです / 寒い, never 寒いだ).
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