Meaning
だ is the plain copula after a noun or na-adjective. It corresponds to polite です but belongs to informal style.
Formation
- Noun + だ: 学生だ; na-adjective stem + だ: 静かだ.
- Do not add だ after an i-adjective: say 高い, not 高いだ.
When to use it
- Informal assertion
- Used with close speakers, in inner speech, and in plain written style.
- Inside a larger sentence
- The plain copula is required before some grammatical endings and in quotations.
Example
この町は静かだ
This town is quiet.
Important notes
- Sentence-final だ can sound firm; casual speech sometimes drops it, especially after a noun.
Common mistakes
- Do not combine the styles as 学生だです.
- Do not place だ after an ordinary i-adjective.