だ

だ is the plain form of です: "is", after a noun or a な-adjective. You use it with friends and family, and in writing that does not address the reader politely – diaries, novels, essays.

Not, was and was not

だ changes the same way です does, and the noun or adjective in front never changes. ではない is the written and more careful form of じゃない.

• がくせい学生だ – is a student
• がくせい学生じゃない – is not a student
• がくせい学生だった – was a student
• がくせい学生じゃなかった – was not a student

な before a noun, だ at the end

A な-adjective takes な when it comes before a noun. At the end of a sentence that な becomes だ, or です when you are polite – never なです.

• × この部屋は静かなです → この部屋は静かです

Not after an い-adjective or a verb

An い-adjective already means "is …", and a verb is complete on its own. Neither takes だ. With an い-adjective, です adds politeness only, and the plain form is the adjective by itself.

• × 高いだ → 高い
• × 行くだ → 行く

• × きれくない → きれいじゃない

Dropping だ in conversation

In casual speech, a bare だ at the end of a sentence sounds blunt. It is often left off, or softened with よ or ね; before ね it can drop as well, and きれいね is softer than きれいだね. A casual question drops it altogether and rises at the end, as in がくせい学生きごう？ "a student?".

• × 今日、暇だ？ → 今日、暇？

Inside a sentence, before けど, から or とおもう思う, だ stays.