固い

1. Origins, structure, and basic meaning

• Structure: 固い is read as かたい and is an i-adjective for physical hardness or a stiff, tense state.
• Basic usage: It describes a hard shell and a facial expression that becomes stiff from tension.

2. Common pitfalls and similar expressions

• Physical property
  • 殻が固い describes a shell as hard and resistant to easy deformation.
• Adverbial form
  • In 表情が固くなる, adverbial 固く combines with なる to express a change into stiffness.