The difference between です (desu) and だ (da) is fundamental in Japanese, because it relates to the level of politeness and the communication setting.

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🟩 1. Same grammatical function

です and だ have the same grammatical function:
→ they are copulas (or "state verbs") used to link a subject to a description or an identity.
In English, they can both be translated by "to be".

Examples

💬 私は学生です。
わたしはがくせいです。
Watashi wa gakusei desu.
→ I am a student.

💬 私は学生だ。
わたしはがくせいだ。
Watashi wa gakusei da.
→ I am a student.

👉 The meaning is the same, but the tone changes.

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🟨 2. The main difference: the level of politeness

Form	Level	When to use	Example
です (desu)	polite	when speaking to a teacher, a superior, or someone you barely know	先生はやさしいです。 せんせいはやさしいです。 The teacher is kind.
だ (da)	neutral / casual	among friends, with family, or in a personal diary	学生だ。 がくせいだ。 He/she is a student.

⚠️ Caution: you cannot say 先生はやさしいだ ❌
("だ" is not used with い-adjectives.)

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🟦 3. Mind the grammar depending on the word type

With a noun or a な-adjective

→ You can use です or だ depending on the level of politeness.

Type	Polite	Neutral
Noun	学生です (gakusei desu)	学生だ (gakusei da)
な-adjective	きれいです (kirei desu)	きれいだ (kirei da)

With an い-adjective

→ You never use だ.
The polite form adds です, and the neutral form keeps the adjective alone.

Type	Polite	Neutral
い-adjective	たかいです (takai desu)	たかい (takai)

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🟧 4. Quick summary

Form	Politeness	Word type	Example	Translation
です	polite	noun / な-adj / い-adj	これはほんです。	This is a book.
だ	neutral	noun / な-adj	これはほんだ。	This is a book.

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🟪 5. In practice

• At school, with a teacher:
  → です

  わたしはフランスじんです。
  I am French.

• With a friend:
  → だ

  おれはフランスじんだ。
  I'm French (casual, masculine form).