暗い

1. Origins, structure, and basic meaning

• Structure: 暗い (くらい) is an adjective for low light or a gloomy expression and atmosphere.
• Basic usage: 暗くなった describes the forest losing light after sunset; 暗い表情 describes a person's somber appearance after hearing a result.

2. Common pitfalls and similar expressions

• Loss of light and gloomy facial expression
  • Inflected 暗くなった is a physical state change; attributive 暗い modifies an expression revealing emotion. The second is not about illumination.