I-adjectives change their 「い」ending to conjugate into 4 forms. Basic: tanoshii (fun) Negative: tanoshikunai (not fun) Past: tanoshikatta (was fun) Neg. past: tanoshikunakatta (was not fun) ✅ Practice: atsui → atsukunai / atsukatta / atsukunakatta ✅ Practice: oishii → oishikunai / oishikatta / oishikunakatta ⚠️ Exception: いい (good) → past form is よかった (NOT いかった)