硬い
かたい · katai · JLPT N3 · i-adj
hard
🪨 硬 has 石 (stone) on its left: as hard as stone, ka-tai
The kanji 硬 is 石 (stone) plus 更, which gives the sound. For sound: katai is like a karate kata, firm and hard. 硬い is for things that are hard to touch, like stone, metal or ice. Its opposite is 柔らかい (yawarakai, soft).
硬 — stone 石 + 更 (gēng, phonetic)
硬 is a semantic-phonetic compound. The left side 石 'stone' gives the meaning—stones are hard—while the right side 更 originally suggested the pronunciation. From the literal sense of stone-hardness, 硬 extended to mean firm, stiff, or stubborn in both physical and figurative uses.