What is だ (da)?
だ (da) is the plain/casual form of the copula, equivalent to です in casual speech. It means "is/am/are" and is used in informal situations, writing, and as the dictionary form of the copula.
A は B だ
"A is B"
私は学生だ
"I am a student"
Basic Usage
With Nouns
これは本だ
"This is a book"
彼は先生だ
"He is a teacher"
私は日本人だ
"I am Japanese"
今日は月曜日だ
"Today is Monday"
With Na-adjectives
この部屋は静かだ
"This room is quiet"
京都はきれいだ
"Kyoto is beautiful"
日本語は便利だ
"Japanese is convenient"
彼は親切だ
"He is kind"
When to Use だ
Casual Situations
Use with friends, family, and close acquaintances.
友達: あれは何だ？
"What's that?"
自分: それは本だ
"That's a book"
Written Language
Common in newspapers, novels, and academic writing.
富士山は日本で一番高い山だ
"Mt. Fuji is Japan's tallest mountain"
これは重要な問題だ
"This is an important problem"
Inner Thoughts
Used when expressing your own thoughts.
今日は暑いな。大変だ
"It's hot today. This is tough"
これは難しいな
"This is difficult"
Quotations and Reports
彼は学生だと言った
"He said he's a student"
これは本当だと思う
"I think this is true"
Question Form
だ + か = か (without だ)
In questions, だ is usually dropped before か in casual speech.
学生か？
"Are you a student?"
日本人か？
"Are you Japanese?"
本当か？
"Is it true?"
Note: だか sounds unnatural. Use just か instead.
Rising Intonation
Questions can be formed with rising intonation alone.
学生？↗
"A student?"
本当？↗
"Really?"
明日？↗
"Tomorrow?"
Negative Form
じゃない (janai)
The casual negative of だ is じゃない.
学生じゃない
"I'm not a student"
本じゃない
"It's not a book"
本当じゃない
"It's not true"
簡単じゃない
"It's not easy"
ではない (dewa nai)
More formal written negative form.
これは本ではない
"This is not a book"
簡単ではない
"It's not simple"
Usage difference:
- じゃない: Spoken, casual
- ではない: Written, formal
Comparing です and だ
Politeness Level
Polite (です): 学生です
Plain (だ): 学生だ
"I am a student"
Polite (です): 日本人です
Plain (だ): 日本人だ
"I am Japanese"
Question Forms
Polite: 学生ですか？
Plain: 学生（だ）？ / 学生か？
"Are you a student?"
Polite: 本当ですか？
Plain: 本当（だ）？ / 本当か？
"Really?"
Negative Forms
Polite: 学生じゃないです / 学生ではありません
Plain: 学生じゃない / 学生ではない
"I'm not a student"
Common Patterns
～だから (because)
学生だから、お金がない
"Because I'm a student, I don't have money"
忙しいんだから
"It's because I'm busy"
日本人だから、日本語が話せる
"Because I'm Japanese, I can speak Japanese"
～だけど (but)
学生だけど、働いている
"I'm a student, but I'm working"
簡単だけど、面白い
"It's simple, but interesting"
～だと思う (I think)
彼は学生だと思う
"I think he's a student"
本当だと思う
"I think it's true"
便利だと思う
"I think it's convenient"
～だって (quotation/emphasis)
彼は学生だって
"He says he's a student / I heard he's a student"
本当だって！
"It's true (I'm telling you)!"
Special Forms
だろう (probably)
Presumptive form of だ.
学生だろう
"He's probably a student"
本当だろう
"It's probably true"
明日だろう
"It's probably tomorrow"
～だった (past)
Past tense of だ.
学生だった
"I was a student"
昨日だった
"It was yesterday"
きれいだった
"It was beautiful"
～だったら (conditional)
学生だったら、割引がある
"If you're a student, there's a discount"
本当だったら、すごい
"If it's true, that's amazing"
In Subordinate Clauses
Before な (na for noun modification)
学生だった人
"A person who was a student"
有名な人
"A famous person"
Before の (nominalizer)
学生なのだ → 学生なんだ
"The thing is, I'm a student"
大変なのだ → 大変なんだ
"The thing is, it's tough"
Common Conversations
Example 1: Casual Chat
A: これ、何？
"What's this?"
B: 辞書だよ
"It's a dictionary"
A: 誰の？
"Whose?"
B: 田中のだ
"It's Tanaka's"
Example 2: Confirmation
A: 明日、休みだよね？
"Tomorrow's a day off, right?"
B: うん、休みだ
"Yeah, it's a day off"
A: 本当？
"Really?"
B: 本当だって！
"It's true!"
Example 3: Explanation
A: どうして来なかったの？
"Why didn't you come?"
B: 忙しかったんだ
"I was busy (that's why)"
A: 仕事？
"Work?"
B: そう、仕事だった
"Yeah, it was work"
Common Mistakes
Mistake 1: Using だ in polite contexts
Wrong (to strangers/superiors): 学生だ ❌
Right: 学生です ✓
"I am a student"
Mistake 2: Using だか in questions
Unnatural: 学生だか？ ❌
Natural: 学生？/ 学生か？ ✓
"Are you a student?"
Mistake 3: Using だ after i-adjectives
Wrong: 大きいだ ❌
Right: 大きい ✓
"It's big"
Note: I-adjectives don't need だ
Mistake 4: Mixing politeness levels
Inconsistent: 私は学生だ。でも、毎日働いています ⚠️
Better: 私は学生だ。でも、毎日働いている ✓
Or: 私は学生です。でも、毎日働いています ✓
Gender Associations
While だ itself is gender-neutral, it's often associated with masculine speech when used in casual conversation. Women might use more です or だわ variations.
Neutral/Masculine
これは本だ
"This is a book"
面白いな
"This is interesting"
Feminine variations
これは本だわ
"This is a book"
面白いわね
"This is interesting"
Note: These distinctions are becoming less strict in modern Japanese.
In Written Japanese
Newspapers
今日は晴れだ
"Today is sunny"
事故の原因は不明だ
"The cause of the accident is unknown"
Novels/Narratives
彼は学生だった
"He was a student"
それは本当のことだ
"That's the truth"
Academic Writing
これは重要な問題だ
"This is an important problem"
その理由は明らかだ
"The reason is clear"
Sentence-ending Particles with だ
だよ (emphasis)
本当だよ
"It's true (I'm telling you)"
学生だよ
"I'm a student (you know)"
だね (agreement)
そうだね
"That's right, isn't it"
便利だね
"It's convenient, isn't it"
だな (contemplative)
難しいな
"This is difficult (hmm)"
そうだな
"That's right (let me think)"
だぞ (strong emphasis - masculine)
本当だぞ
"It's really true!"
危ないぞ
"It's dangerous!"
Practice Sentences
これは私のだ
"This is mine"
彼は学生だ
"He's a student"
それは本当だ
"That's true"
簡単じゃない
"It's not easy"
昨日は月曜日だった
"Yesterday was Monday"
たぶん明日だろう
"It's probably tomorrow"
学生だから、忙しい
"Because I'm a student, I'm busy"
本だけど、面白くない
"It's a book, but it's not interesting"
誰のペンだ？
"Whose pen is this?"
どこだ？
"Where is it?"
Related Grammar Points
- です (desu) - Polite copula
- である (de aru) - Formal copula
- じゃない (janai) - Negative forms
- だった (datta) - Past forms
- Casual Speech Patterns - Informal Japanese
- Sentence-ending Particles - ね, よ, な, etc.