Adjectives
な-adjectives / い-adjectives
Japanese has two families of adjectives — な-adjectives and い-adjectives — that conjugate differently and attach to nouns in different ways.
Two adjective families
Every Japanese adjective falls into one of two groups: な-adjectives or い-adjectives. The two families look similar in English ("kind," "expensive") but conjugate in fundamentally different ways. You'll memorize each adjective with its type so you know which pattern to apply.
な-adjectives — conjugate like nouns
な-adjectives behave grammatically like nouns. As a predicate, attach だ (or its forms じゃない / だった / じゃなかった) just like you would to a noun: 「ボブ は しんせつ だ」 = "Bob is kind." When a な-adjective directly modifies a noun, insert な between them: 「しんせつ な ひと」 = "a kind person." The な is what gives the family its name.
い-adjectives — end in い and conjugate themselves
い-adjectives always end in the hiragana い in their plain positive form. Unlike な-adjectives, they don't take だ as a predicate — the adjective already inflects for tense and polarity. To form the negative drop い and add くない: たかい → たかくない ("not expensive"). For the past, drop い and add かった: たかい → たかかった ("was expensive"). Past negative is くなかった: たかくなかった. When an い-adjective modifies a noun, it attaches directly with no な: 「たかい レストラン」 = "expensive restaurant."
Irregular — いい (good)
いい ("good") is the one common い-adjective whose conjugations are irregular. All its inflected forms come from the older base よい, not from いい. So: いい (good) → よくない (not good) → よかった (was good) → よくなかった (wasn't good). The positive form いい is still used; only the conjugated forms switch to the よ- stem.
Vocabulary in this lesson
About Japanese adjectives — な-adjectives and い-adjectives
Japanese has two families of adjectives, and knowing which family a word belongs to decides how you make it negative, put it in the past, or attach it to a noun. な-adjectives conjugate like nouns — you add だ and its forms. い-adjectives end in the hiragana い and conjugate themselves by swapping that い for くない, かった, and くなかった. One word, いい ("good"), is irregular and conjugates from よ-. Adjective conjugation is core JLPT N5 grammar, and the same く / past patterns come back when you build adverbs later.
Frequently asked questions
What are the two types of Japanese adjectives?
Japanese adjectives split into two families: な-adjectives and い-adjectives. They describe things the same way but conjugate differently, so the first skill is telling them apart.
How can I tell a な-adjective from an い-adjective?
い-adjectives end in the hiragana い in their plain form (たかい, おいしい). Most others are な-adjectives (きれい, しずか). Watch out: a few な-adjectives such as きれい also end in い, so the ending is a strong hint rather than an absolute rule.
How do な-adjectives conjugate?
They behave like nouns: attach だ / じゃない / だった / じゃなかった for the four plain forms, and add な directly before a noun — 「しずか な へや」 means "a quiet room."
How do い-adjectives conjugate?
Drop the final い and add the ending: negative くない, past かった, past-negative くなかった. So たかい becomes たかくない (not expensive) and たかかった (was expensive). They never take だ.
Why is いい (good) irregular?
いい conjugates from its older form よい, so every inflected form uses よ-: よくない (not good), よかった (was good), よくなかった (wasn't good). Only the plain positive stays いい.
Credit
These lessons follow the structure, sequence, and grammar-first approach of Tae Kim's Guide to Learning Japanese. We are grateful to Tae Kim for making his guide freely available; the explanations here are our own retelling, and the interactive drills are original to this site.