Iadjectives

I-adjectives, or i-adjectives, are a class of adjectives in Japanese grammar whose dictionary form ends in the syllable -い. They describe properties or states of nouns and modify nouns directly, as in 高い山 “tall mountain” or 安い本 “cheap book.” They can also predicate a statement, as in 山は高いです “The mountain is tall.”

Conjugation and usage follow regular patterns for tense and negation. The present affirmative form is the dictionary form ending in い. To form the negative, replace the final い with くない (高い → 高くない). The past affirmative is -かった (高い → 高かった); the past negative is -くなかった (高くなかった). The adverbial form is -く (高く歩く). The te-form is -くて (高くて). These forms apply to most i-adjectives.

There is an important irregular: いい “good.” Its more formal or literary dictionary form is よい, and its conjugations follow the よい pattern in many contexts: よい → よくない, よかった, よくなかった, よくて. In everyday speech, people often use いい with corresponding negative, past, and te-form constructions (いいです, よくない, よかった, よくて).

Differences from na-adjectives are notable. i-adjectives modify nouns directly without any particle: 高い山. Na-adjectives require the particle な when modifying a noun (静かな山). In predicative position, i-adjectives can take polite endings like です/でした (高いです), while na-adjectives take だ/です after the noun phrase (静かだ, 静かでした). Adverbs are formed by replacing -い with -く (速い → 速く).