In short

Written Japanese has a third register beside です／ます and casual だ: である体 de aru tai, the style of essays, reports, papers and editorials. Switching into it is a package deal - the copula becomes である, clauses are linked with a bare ます-stem instead of the て-form, negatives become 〜ず, and the connectives change (でも -> しかし, だから -> したがって). Pick one style per text and never mix them.

The rule

Step 1 - Three styles, chosen by the text, not by the topic. Verbs and い-adjectives look identical in the two plain styles; what really separates them is the copula and the vocabulary around it.

• です／ます体 (teineitai) - anything addressed to a person: speech, letters, business email, customer-facing writing. This is still your default (lesson 44).
• だ体 (futsūtai) - casual writing with no distance: diaries, blogs, personal notes, novels.
• である体 (de aru tai) - writing with no addressee: academic papers, reports, editorials, encyclopedia entries, exam essays (小論文 shōronbun).

である is not more polite than です - it is less interpersonal. It sounds objective and impersonal precisely because it does not acknowledge a reader. Never write it in an email to a human being.

Step 2 - The である paradigm. である is literally で + ある, so it inflects like the verb ある, and its negative is ではない de wa nai - you do not stack a negative onto である. The explanatory 〜んです of lesson 20 has its own written shape, 〜のである no de aru (“the fact is that…”), with 〜のであろう for a conjecture; the な-insertion after a noun or な-adjective survives (学生なのである). であろう de arō is the written でしょう／だろう, and in formal prose it attaches to verbs too: 増加するであろう zōka suru de arō (“will presumably increase”).

Step 3 - 連用中止法: link clauses with the ます-stem. In speech you join clauses with the て-form (lesson 13). In writing you can cut the て off and leave the bare ます-stem (連用形 ren’yōkei). The meaning is the same; the rhythm is tighter and more formal.

朝六時に起きて、七時に家を出た。 -> 朝六時に起き、七時に家を出た。

i-adjectives use their く-form (安くて -> 安く), and nouns and な-adjectives use であり (学生で -> 学生であり). 連用中止 is the standard way to write a list of two or three parallel facts: put the stems in the middle links and let the last predicate carry the tense for all of them. Note that 連用中止 is a loose “and”, just like て: whether the link is a list, a sequence or a cause is read from context (大雨が降り、試合は中止となった, heavy rain fell and the match was called off).

Step 4 - The negative 連用中止: 〜ず and adnominal 〜ぬ. 〜ず zu is the written negative link - it does the job of ないで／なくて. Build it from the ない-base and replace ない with ず: 行かない -> 行かず, 食べない -> 食べず. The one irregular is する -> せず sezu (never ×しず); 来る follows the rule: 来ない -> 来ず kozu. As a clause link: 何も言わず、部屋を出た。 (Nani mo iwazu, heya o deta.) - He left the room without saying anything.

• 〜ずに is the same form used adverbially, “without doing”; it is fully alive in ordinary speech and is owned by lesson 54.
• 〜ぬ nu is the adnominal (noun-modifying) partner of ず, surviving mostly in frozen items: 思わぬ結果 omowanu kekka (“an unexpected result”), 見知らぬ人 mishiranu hito (“a stranger”). Recognise these; do not coin new ones. Literary 〜ずして (“without even doing”, 戦わずして勝つ tatakawazu shite katsu) is C1 material - just note that it exists.

Step 5 - 〜べきだ: the written “should”. V-dictionary + べき states an obligation from a principle rather than from circumstances (contrast 〜なければならない, lesson 35): 規則は守るべきである。 (Kisoku wa mamoru beki de aru.) - Rules should be obeyed.

• Negative 〜べきではない beki de wa nai; past regret 〜べきだった (“should have”).
• Before a noun, べき attaches directly, with no の: 守るべき規則 mamoru beki kisoku. After a noun, insert である: 学生であるべきだ.
• With する both するべき and すべき su beki are correct; すべき is the more formal, more written of the two.

Step 6 - Formal connectives. The spoken connectives of lesson 22 have written counterparts, and swapping them is half of a style conversion. They stand at the head of a sentence, usually followed by 、 - see the table below.

Step 7 - Nominalization-heavy style. Formal Japanese prefers a Sino-Japanese noun where speech uses a verb clause: 人口が増えること -> 人口の増加 jinkō no zōka, 物価が上がったので -> 物価の上昇により bukka no jōshō ni yori. The whole sentence gets shorter and the の-chain carries the structure.

Step 8 - The exercise that trains all of this: convert a passage both ways. Take four polite sentences and rewrite them as である体, then back. Change the copula, the connectives, the negatives, the clause links and the vocabulary together - a half-converted text is the classic failure.

• です／ます体: 昨日、新しい図書館に行きました。とてもきれいで、静かでした。でも、本があまり多くありませんでした。だから、何も借りないで帰りました。
• である体: 昨日、新しい図書館を訪れた。館内は美しく、静かであった。しかし、蔵書はあまり多くなかった。したがって、何も借りずに帰った。 (Kinō, atarashii toshokan o otozureta. Kannai wa utsukushiku, shizuka de atta. Shikashi, zōsho wa amari ōkunakatta. Shitagatte, nani mo karizu ni kaetta.)

Forms & patterns

The three styles side by side.

Linking clauses: て-form vs 連用中止 vs ず.

〜ず and 〜ぬ by verb class.

Formal connectives.

Conversion checklist: です／ます体 -> である体.

Examples

Common mistakes

• 私は学生であります。 (Watashi wa gakusei de arimasu.) -> 私は学生である。 (Watashi wa gakusei de aru.) - である belongs to a plain-style text, so it never takes ます. であります survives only in old political and military speech; in modern writing it reads as a costume.
• この問題は難しいである。 (Kono mondai wa muzukashii de aru.) -> この問題は難しい。 (Kono mondai wa muzukashii.) - an い-adjective is already a complete predicate. である attaches only to nouns and な-adjectives, exactly where です would stand.
• これは問題であるではない。 (Kore wa mondai de aru de wa nai.) -> これは問題ではない。 (Kore wa mondai de wa nai.) - the negative of である is ではない; you replace ある, you do not negate it a second time.
• 勉強しずに試験を受けた。 (Benkyō shizu ni shiken o uketa.) -> 勉強せずに試験を受けた。 (Benkyō sezu ni shiken o uketa.) - する has the irregular ず-form せず. This one error is worth more drilling than the whole rest of the paradigm, because する compounds are everywhere.
• 知るべきの情報である。 (Shiru beki no jōhō de aru.) -> 知るべき情報である。 (Shiru beki jōhō de aru.) - べき modifies a noun directly. It is not a noun itself, so no の comes between them.

Tips

• One text, one style - check the last character of every sentence. A paragraph whose sentences end である。 である。 and then です。 shows the single most visible mistake in a Japanese essay, and it is invisible to you unless you scan the sentence endings deliberately. Do that scan as a final pass.
• である is impersonal, not polite. Ask who is being addressed: a person -> です／ます; a reader-less document -> である. This is why a report can be である体 inside while its covering email is です／ます体.
• Half a conversion is worse than none. Changing ました to た but leaving でも, 〜ないで and 〜んです in place produces a text that reads as a broken essay rather than a friendly one. Convert the copula, connectives, negatives and clause links in the same pass - that is exactly what Step 8 trains.
• Two ず forms you already meet in speech. 思わず omowazu (“involuntarily, in spite of oneself”) and 相変わらず aikawarazu (“as ever”) are frozen ず-forms in everyday use - proof that this “literary” ending is closer than it looks.