The Difference Between「〜です」と「〜だ」のフォーマル度の違いの違い｜Japanese Grammar (N5–N3)

(Joy speaking politely with a boss using ~desu [Professional] vs. Ken chatting casually with a friend using ~da [Casual])

“This is a book.” (To a teacher) -> Kore wa hon desu. (Polite) “This is a book.” (To a friend) -> Kore wa hon da. (Casual) At their core, both 〜です (~desu) and 〜だ (~da) mean “it is” or “I am.” However, Japanese grammar is deeply tied to social hierarchy. Using the wrong form doesn’t just make your sentence grammatically incorrect—it can actually offend the person you are talking to! Let’s watch what happens when Joy mixes up her formality levels at the office.

🗣 Conversation Scene

Joy is handing a very important report to Director Tanaka.

Joy

ディレクター、明日の会議のレポートだ！ (Director, here “da” [it is] the report for tomorrow’s meeting!)

Tanaka

……レポート「だ」？私たちは友達じゃないですよ、Joyさん。 (…Report “da”? We are not friends, Joy. *Sighs*)

[ Ken quickly helps Joy correct her mistake. ]

Ken

Joyさん、会社では「です」を使いましょう！「だ」を使うと、タメ口（友達言葉）になってしまいます！ (Joy, you must use “desu” at the company! If you use “da”, it becomes “tameguchi” [casual friend talk]!)

💡 Explanation

The Wall of Politeness

⚠️ The Golden Rule of Desu vs. Da: 〜です (~desu) puts up a polite wall between you and the listener. 〜だ (~da) tears that wall down completely.

1. 〜です (~desu) = Polite / Professional 🏢

Target: Strangers, Coworkers, Bosses, Staff. This is the standard “Polite Form” (Teineigo) taught in textbooks. It shows that you respect the social distance between yourself and the listener. As a foreign learner, you should use `~desu` 90% of the time, especially when you are out in public or at work.

[To a store clerk] 私はアメリカ人です。 (I am American. *Polite*)
[To a boss] 明日は火曜日です。 (Tomorrow is Tuesday. *Professional*)

2. 〜だ (~da) = Casual / Intimate 🏠

Target: Family, Close Friends, Talking to yourself. This is the “Plain Form” (Short Form). It completely removes the social wall. While it is great for building intimacy with your best friends, using it with someone older or of higher status sounds like you are treating them like a child or a drinking buddy.

[To your best friend] 私はアメリカ人だ。 (I am American. *Casual*)
[To yourself/muttering] あ、明日は火曜日だ。 (Ah, tomorrow is Tuesday. *Talking to oneself*)

📊 Comparison Table

Word	Formality Level	Who to use it with
〜です (~desu)	Polite (Long Form) Maintains a respectful wall.	Superiors, strangers, teachers, coworkers.
〜だ (~da)	Casual (Short Form) Zero boundaries.	Best friends, family members, or talking to yourself.

🚧 The “News Article” Trap

If you read a Japanese newspaper or a Wikipedia article, you will notice that almost every sentence ends in 〜だ (~da) or 〜である (~dearu). This does not mean the writer is being rude! In written Japanese, the plain form is used to state objective facts. However, in spoken conversation, using “~da” carries the strong emotion of casual intimacy.

🔥 Practice Quiz

Tap to check the answer!

Q1. You are asking a police officer for directions. You want to point to a building and say, “Is it that building?” A. Ano tatemono da? B. Ano tatemono desu ka?

Answer: B (~desu ka?) A police officer is a stranger, so you must use the polite form.

Q2. You are having drinks with your best friend from college. You want to say, “I am sleepy.” A. Nemui da. (Or just “Nemui”) B. Nemui desu.

Answer: A (Casual Form) Using “desu” with your best friend while drinking will make you sound very stiff and distant!

Keep practicing with JPLT!