You i-adjectives in Japanese, also called い adjectives, can appear directly before a noun and also end a sentence. To inflect them, the main rule is to remove the final い and add the necessary ending: ～くない in the negative, ～かった in the past, and ～くなかった in the negative past.
This transformation belongs to the adjective itself. です can make the sentence polite, but it should not be used as if it were the only part responsible for tense or negation.
What are i-adjectives in Japanese?
い-adjectives describe qualities, states, sensations, or evaluations. The basic form usually ends in い, like 高い (takai, tall or expensive), 新しい (atarashii, new), and おもしろい (omoshiroi, interesting).
They have two main uses:
- Before a noun: 新しい本 (atarashii hon) — new book.
- As a predicate: この本は新しいです (Kono hon wa atarashii desu) — This book is new.
Not every word that graphically ends in い belongs to this group. きれい (kirei, beautiful or clean), and 有名 (yūmei, famous), for example, are な adjectives. If in doubt, check the word class in the dictionary and consult the general guide to adjectives in japanese.
The four main forms of i-adjectives
Instead of memorizing “sixteen inflections,” start by crossing two choices: non-past or past and affirmative or negative. The polite form can be added later.
| Function | Plain form | Common polite form | Example with 高い |
|---|---|---|---|
| Non-past affirmative | ～い | ～いです | 高い／高いです |
| Non-past negative | ～くない | ～くないです／～くありません | 高くない／高くないです |
| Affirmative past | ～かった | ～かったです | 高かった／高かったです |
| negative past | ～くなかった | ～くなかったです／～くありませんでした | 高くなかった／高くなかったです |
The formation chart for the plain style and the polite style in the Irodori da Japan Foundation records these four oppositions. The introductory material also presents ～くないです and the alternative ～くありません.
How to form the negative and the past
Negative: replace い with くない
高い → 高くない
たかい → たかくない
Takai → takakunai
It is expensive/tall → It is not expensive/tall.
この店は高くないです。
この みせは たかくないです。
Kono mise wa takakunai desu.
This store is not expensive.
Affirmative past: replace い with かった
おもしろい → おもしろかった
Omoshiroi → omoshirokatta
It is interesting → It was interesting.
映画はおもしろかったです。
えいがは おもしろかったです。
Eiga wa omoshirokatta desu.
The movie was interesting.
Negative past: replace い with くなかった
忙しい → 忙しくなかった
いそがしい → いそがしくなかった
Isogashii → isogashikunakatta
It is busy → It was not busy.
昨日は忙しくなかったです。
きのうは いそがしくなかったです。
Yesterday I was not busy.
Yesterday I was not busy.
Note that it was expensive and it was interesting are not the normal past forms. The past appears in the adjective: 高かったです and it was interesting. To review the role of です without treating it as a perfect equivalent of “to be”, see the article on です in Japanese.
The exception いい: use the base よ in inflections
いい means “good” and originates from よい. In the basic form, いい is very common; in other forms, use the base よ:
| Function | Correct form | Form to avoid |
|---|---|---|
| Non-past affirmative | いい／よい | — |
| Non-past negative | よくない | いいくない |
| Affirmative past | よかった | いいかった |
| negative past | よくなかった | いいくなかった |
The Japan Foundation material on past impressions highlights that いい forms the past as よかったです. The same pattern applies to compounds ending in いい, such as かっこいい → かっこよかった.
How to connect adjectives and indicate change
Use ～くて to connect characteristics
Replace the final い with くて to connect an い adjective to the rest of the sentence:
この部屋は広くて、明るいです。
この へやは ひろくて、あかるいです。
Kono heya wa hirokute, akarui desu.
This room is spacious and bright.
THE Japan Foundation presents ～くて as a way to connect い adjectives. Depending on the context, the first characteristic can also function as a cause or explanation for the second part.
Use ～くなる for “to become” or “to get”
With なる, the ～く form expresses change:
だんだん寒くなりました。
だんだん さむくなりました。
Dandan samuku narimashita.
It gradually got colder.
The pattern い adjective without the final い + くなります is used by Irodori to describe changes in weather. The same ～く base can also modify an action: 名前を大きく書いてください (Namae o ōkiku kaite kudasai) means “Please write your name in large letters.”.
Essential difference between i and na adjectives
Before nouns, the い adjective keeps the い: 静かな部屋 is “quiet room” with a な adjective, while Spacious room is “spacious room” with the adjective い. In the predicate and inflections, each group follows its own pattern.
| Usage | い adjective | な adjective |
|---|---|---|
| Before the noun | Expensive store | Quiet store |
| Polite predicate | The store is expensive | The store is quiet |
| Simple negative | Not expensive | It is not quiet |
The article on な adjectives in Japanese will be the next deep dive of the cluster. To build complete sentences with these forms, also use the method of Japanese sentence formation.
Common mistakes with i adjectives
- Calling the form in ～い an infinitive: “basic form” or “non-past affirmative form” is more accurate.
- Treating every word ending in い as an い adjective: きれい and 有名 are な adjectives.
- Making the past with でした: use 高かったです, not 高いでした.
- Inflecting いい like a regular adjective: use よくない, よかった and よくなかった.
- Removing the い before a noun: say 新しい本, not 新し本.
Short dialogue
A: How was that restaurant?
How was that restaurant?
Sono mise wa dō deshita ka?
How was the restaurant?
B: The food was delicious. But the restaurant wasn't very spacious.
The food was delicious. But the restaurant wasn't very spacious.
Ryōri wa oishikatta desu. Demo, mise wa amari hirokunakatta desu.
The food was tasty. But the restaurant wasn't very spacious.
A: Was the price high?
Was the price high?
Nedan wa takakatta desu ka?
Was the price high?
B: No, it was cheap.
No, it was cheap.
Iie, yasukatta desu.
No, it was cheap.
The dialogue combines affirmative past and negative past without shifting the inflection to です. If the order of elements still causes doubt, review the Basics of Japanese Grammar.
Commented exercise
- 高い in the non-past negative: Not expensive.
- 楽しい in the affirmative past: 楽しかった.
- 忙しい in the negative past: 忙しくなかった.
- いい in the affirmative past: よかった.
- Una Wide and Bright: Wide and bright.
The mental rule is simple: keep the い in the basic form and before nouns; to transform the adjective, replace that い with くない, かった, くなかった or くて. Then, confirm whether the word is really an i-adjective and remember the base よ of いい.
Is every adjective ending in い an i-adjective?
No. The graphical ending helps, but words like きれい and 有名 belong to the group of な adjectives. When in doubt, confirm the class in the dictionary and observe how the word appears before a noun.
How to make an adjective negative in form?
Remove the final い and add くない. 高い becomes 高くない; in the polite form, use 高くないです or 高くありません.
How do you put an adjective in the past tense?
Remove the final い and add かった: おもしろい becomes おもしろかった. In the polite form, say おもしろかったです, not おもしろいでした.
What is the difference between i-adjectives and na-adjectives?
い-adjectives keep the い before the noun and inflect into forms like くない and かった. な-adjectives use な before the noun and follow a different pattern in the predicate, such as 静かです and 静かじゃない.
Why does いい become よかった?
It preserves a historical base linked to よい. Therefore, its derived forms use よ: よくない, よかった, and よくなかった. Compounds like かっこいい follow the same pattern.