改まる

1. Origins, structure, and basic meaning

• Structure: 改まる is read as あらたまる and is an intransitive verb for a rule becoming revised or an atmosphere or manner becoming formal.
• Basic usage: The changing item is the が subject in patterns such as 規則が改まる and 雰囲気が改まる.

2. Common pitfalls and similar expressions

• Direction versus 改める
  • 改める is transitive: someone changes a rule or manner. 改まる presents the rule or manner as undergoing the change.
• Choosing the sense
  • Rules and systems suggest revision; atmosphere, speech, or dress suggests a shift to formality.