べからず / べからざる

べからず and べからざる are classical Japanese expressions that convey strong prohibition or negation. They are the negative forms of the classical auxiliary べし ("should" / "must"), combined with the classical negative auxiliary ず (which becomes ざる when modifying a noun).

べからず – "must not" / "do not"

べからず attaches to the dictionary form of a verb and expresses a strong prohibition, meaning "must not" or "do not." It is commonly found on signs, rules, proverbs, and formal notices rather than in everyday conversation.

Because of its classical and authoritative tone, べからず sounds much more forceful than modern expressions like 〜～てはいけない or 〜～てはならない. It carries an impersonal, rule-like quality – as if the prohibition applies universally, not just to one person.

べからざる – "must not (be)" / "that should not exist"

べからざる is the attributive (noun-modifying) form of べからず. It is used to modify a noun, expressing that something is "unacceptable," "impermissible," or "that which must not be."

A very common set phrase is かく欠くべからざる, meaning "indispensable" or "that which must not be lacking":

Key differences

• べからず stands at the end of a sentence or phrase (predicative use).
• べからざる modifies a noun that follows it (attributive use).

Notes

• Both forms are literary and archaic in tone. They appear in written language, formal regulations, proverbs, and set expressions, but are rarely used in casual speech.
• The verb before べからず / べからざる is always in the dictionary form (終止形).
• A well-known proverb:
  しょしん初心わする忘るべからず。

  One must not forget one's original intent.