Grammar · N5
です and だ: the copula
Identify or classify nouns and な-adjectives with polite and plain forms.
In short
です is a polite copular form; だ is a common plain form used with nouns and な-adjectives in appropriate contexts.
How it works
学生です is polite; 学生だ is plain. In many subordinate structures the plain form is required, and nouns/な-adjectives may use な or で depending on what follows.
Useful contrast
です is not simply a Japanese translation of English “to be” in every sentence. Adjectives and verbs already form predicates without it in their own ways.
Examples
私は学生です。
I am a student.
ここは静かだ。
It is quiet here.
Common pitfall
Do not attach だ directly after an い-adjective: *高いだ is wrong. 高い is already a plain predicate.