Plain written style

The だ・である調 (also known as 常体（じょうたい）) is essential for formal writing. While it utilizes plain forms, it is distinct from casual speech due to the inclusion of the literary copula である.

1. The Golden Rule:
In formal writing, never mix the だ・である調 (plain style) with the ですます調 (polite style) in the same text, with the exception of direct quotations (inside 「」) and bulleted lists (箇条書き). Bulleted lists often use plain forms or noun-endings for brevity even within a polite text. Otherwise, mixing these styles is considered a major grammatical error in essays and professional writing, though professional writers may intentionally mix them in literature or personal essays.

2. Usage Contexts:

• Academic Writing: Theses and research papers almost exclusively use である.
• Journalism: Newspapers frequently use both だ and である (especially in editorials).
• Literature: Novels use this style for narration.
• Professional Records: Professional reports and official documents.
• Personal Writing: Personal diaries often use the plain form (だ), though writers may intentionally use である to adopt a philosophical, objective, or dramatic internal monologue tone.

3. Morphological Rules:

• Nouns & な-Adjectives: Attach だ (direct assertion) or である (formal/literary assertion). While だ overlaps with casual speech, である is overwhelmingly a written style and is almost never used in spontaneous spoken Japanese, even in formal settings, unless reading directly from a prepared text.
• Verbs & い-Adjectives: Use all plain forms, including the dictionary form (書く（かく）), negative form (書かない（かかない）), past form (書いた（かいた）), and negative past form (書かなかった（かかなかった）). Do not attach だ directly to verbs or い-adjectives. If you need to attach だ or である to a verb or い-adjective for explanatory emphasis, you must use the nominalizer の (e.g., 書くのだ, 高いのである).
• Conjecture: In formal writing, the plain だろう is often replaced by the more formal であろう.
• Negative/Past: Use plain forms like ではない, だった, or であった.
• Questions: Do not use だか or であるか directly. To form a question, drop the copula and use か (e.g., 本当か。), use のか (e.g., 本当なのか。), or use the formal であろうか。 Note that in formal writing, the question mark symbol (？) is generally prohibited, and questions must end with a standard Japanese period (句点).

4. Consistency:
While mixing the だ・である調 with the ですます調 is strictly forbidden (outside of quotes and lists) in formal writing, mixing だ and である within the same text is grammatically acceptable and extremely common in general writing to avoid repetitive endings and adjust rhythm. However, in strict formal writing (such as academic papers and official documents), mixing them is often penalized, and you are required to unify the text entirely to である.

5. The Continuative Form (であり):
A fundamental feature of this style is the use of であり to link sentences. It functions as the formal, written equivalent of the て form for nouns and な-adjectives (e.g., 彼は学生であり、研究者でもある。). This allows for a more sophisticated flow in academic prose compared to using the て conjunction.

6. Logical Reasoning (〜のである):
A core feature of this style is the use of 〜のである (or 〜のだ). This functions similarly to the explanatory んです but is used in formal contexts to provide evidence, explain causes, or present a logical conclusion (e.g., 影響（えいきょう）を及（およ）ぼすのである). Note that the colloquial んだ or んである must be strictly avoided in formal writing; always write them out fully as のだ or のである.

7. Nuance Differences & Noun Modification:
While both are used in writing, である feels more analytical and formal than だ. Crucially, while である can function as a noun modifier (e.g., 学生（がくせい）である彼（かれ）), だ CANNOT directly modify nouns (e.g., 学生だ彼は is ungrammatical; it must be 学生の彼 or 学生である彼).

8. Noun-Ending (体言止め):
In journalism and professional reports, sentences often end directly with a noun (体言止め（たいげんどめ）) rather than だ or である. This technique is used for brevity, impact, and to adjust the rhythm of the text.

9. Honorifics in Plain Style:
Even when using the plain だ・である調 for sentence endings, you can and should still use honorific language (尊敬語/謙譲語) within the sentence to describe the actions of respected figures (e.g., 教授（きょうじゅ）がおっしゃったように〜である。). The sentence ending remains plain, but the internal vocabulary maintains respect.