余計

1. Origins, structure, and basic meaning

• Structure: 余計 (よけい) can mark something unnecessary or an amount beyond a baseline. 余計な modifies a noun; 余計に modifies the amount of an action.
• Basic usage: 余計な口出し is unwanted interference, whereas 一時間余計に歩いた adds an extra hour to the amount walked.

2. Common pitfalls and similar expressions

• Unwanted involvement versus excess amount
  • The “unnecessary” sense of 余計な and the “extra” sense of 余計に are not interchangeable. The following noun or quantified action selects the reading.