難問

1. Origins, structure, and basic meaning

• Structure: 難問 (なんもん) is a question or challenge that is hard to answer or solve. It appears as an individual's problem and a long-standing collective task.
• Basic usage: One person encounters a problem unsolved after hours; experts tackle an issue that has persisted for years.

2. Common pitfalls and similar expressions

• Immediate hard problem and long-standing unresolved challenge
  • 解けない難問 focuses on solvability, while 長年の難問 emphasizes how long it has resisted resolution. It does not merely mean a long question.