Doing as you please is strictly forbidden with 〜べからず
Grammar Explanation: Strongly forbidden with 〜べからず

Sazanka sneaks out of class, leaving the teacher to send Asagao to bring her back. As Asagao chases Sazanka across the rooftops near the upper grade classroom, Tsubaki spots them. She meets them up on the roof and recites the school’s first rule.
- ツバキ:
- 「ひとつ。無断で勝手
( な行動( をとるべからず。」- “One: Doing as you please without permission is strictly forbidden.”
- Literal: “One. Taking selfish action without permission is not to be done.”
- “One: Doing as you please without permission is strictly forbidden.”
Key Points
1べからず = “must not / is strictly forbidden”
- Here とるべからず gives the line the feel of an official rule: “must not take such action” / “is strictly forbidden”.
- Pattern: [verb dictionary form] + べからず “must not [verb]”.
- A natural modern paraphrase here is 無断
( で勝手( な行動( をとってはいけない.
2What is being forbidden: 無断( で勝手( な行動( をとる
- The whole phrase before べからず describes the prohibited behavior: taking self-directed action without permission.
- 無断
( で = “without permission, without authorization”. - 勝手
( な行動( = “self-willed / arbitrary behavior”; not just any action, but acting on your own judgment.
3Sentence anatomy: [無断( で] [勝手( な行動( ] をとる
- 無断
( で modifies how the action is done, while 勝手( な modifies 行動( . - A simplified core is 行動
( をとる “to take action / engage in behavior”. - So the line builds from the noun phrase outward: unauthorized + self-directed + action.
4Formal rule-book tone: ひとつ。〜べからず。
- ひとつ。 introduces item one in a formal list of rules or precepts.
- Together with べからず, it makes Tsubaki sound like she is reciting a fixed code rather than speaking casually.
- That stiff, old-fashioned style matches the school-rule atmosphere of the scene.
See Also
くノ一ツバキの胸の内 © its respective creators. It is used here for educational commentary.