Japanese Adjective Conjugator

Japanese has two grammatical categories of adjectives. I-adjectives end in the hiragana い (i) in their dictionary form, such as 大きい (ookii, big) or 寒い (samui, cold), and they conjugate by changing their final い ending. Na-adjectives, sometimes called adjectival nouns, do not conjugate themselves; instead they attach to nouns using な (such as 静かな部屋, a quiet room) and form their tenses by adding the copula です or its plain forms だ, だった, ではない. The distinction matters because they follow completely different conjugation rules — making the wrong choice is one of the most common beginner mistakes.

The adjective いい (ii, meaning good) is irregular because it borrows all of its conjugated forms from an older adjective よい (yoi). The dictionary form can be either いい or よい, but every other conjugation must use the よ stem: the negative is よくない (yokunai), the past is よかった (yokatta), and the te-form is よくて (yokute). This is why beginners hear よかった constantly to mean it was good or thank goodness, even though they learn いい first. The irregularity also carries into compound adjectives such as かっこいい (cool), which becomes かっこよかった in the past tense.

For i-adjectives, drop the final い and add くて: 大きい (ookii) becomes 大きくて (ookikute), 寒い (samui) becomes 寒くて (samukute). The irregular いい becomes よくて. For na-adjectives, simply add で after the adjective stem: 静か (shizuka) becomes 静かで (shizuka de), 元気 (genki) becomes 元気で (genki de). The te-form is used to connect adjectives to other clauses, list multiple qualities (this room is quiet and clean), or give reasons (because it was cold). Mastering the te-form is essential because it is one of the most frequently used grammar structures in everyday Japanese conversation.

For i-adjectives, drop the final い and add く: 早い (hayai, fast) becomes 早く (hayaku, quickly), 上手い (umai, skillful) becomes 上手く (umaku, skillfully). The irregular いい becomes よく (yoku, well or often). For na-adjectives, simply add に: 静か (shizuka) becomes 静かに (shizuka ni, quietly), 簡単 (kantan, easy) becomes 簡単に (kantan ni, easily). Adverb forms are used to modify verbs and describe how an action is performed — they are essential for natural-sounding Japanese sentences.

The four most frequent mistakes are: First, conjugating na-adjectives like i-adjectives, such as saying 静かくない instead of the correct 静かではない. Second, treating きれい (kirei, pretty), 嫌い (kirai, hateful), and 有名 (yuumei, famous) as i-adjectives because they end with the い sound — they are actually na-adjectives. Third, forgetting that いい becomes よく forms in conjugation, producing incorrect forms like いくない or いかった. Fourth, attaching です to an i-adjective in the past tense incorrectly, saying 寒いでした instead of the correct 寒かったです. Practicing with a conjugation chart helps internalize these patterns.

JLPT N5 requires the basic four forms for both i and na adjectives: present affirmative, present negative, past affirmative, and past negative, in both plain and polite styles. JLPT N4 adds the te-form for connecting clauses, the adverb form, and conditional forms such as たら and ば. JLPT N3 expects fluent use of all the above plus comparative and superlative constructions, and adjective + そう (looks like), すぎる (too much), and なる (becomes) patterns. JLPT N2 and N1 focus less on basic conjugation and more on advanced patterns and nuanced vocabulary, so mastering the conjugations early frees you to focus on grammar at higher levels.