A1 形容詞 1 min read かんたん

い-Adjectives: Japanese Adjectives That Conjugate

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Grammar Rule in 30 Seconds

I-adjectives are special because they conjugate like verbs to show tense and polarity by changing their final 'i'.

  • Present affirmative: Keep the 'i' (e.g., {暑い|あつい} desu).
  • Present negative: Drop 'i' and add 'kunai' (e.g., {暑く|あつく}ない).
  • Past affirmative: Drop 'i' and add 'katta' (e.g., {暑かった|あつかった}).
Adjective(i) - i + [Ending] = Conjugated Adjective

Overview

Japanese has two types of adjectives, and い-adjectives (i-adjectives) are the ones that end in い and can conjugate on their own — meaning they change form for negative, past, and past negative without needing any helper words. Common い-adjectives include: 大きい (ookii, big), 小さい (chiisai, small), 高い (takai, expensive/tall), 安い (yasui, cheap), 美味しい (oishii, delicious), 暑い (atsui, hot), 寒い (samui, cold), 新しい (atarashii, new), 古い (furui, old), 面白い (omoshiroi, interesting). To use an い-adjective before a noun, simply place it directly: 大きい家 (big house), 美味しい寿司 (delicious sushi).
To make a sentence: この寿司は美味しいです (This sushi is delicious). For conjugation, drop the final い and add: くない for negative (美味しくない = not delicious), かった for past (美味しかった = was delicious), くなかった for past negative (美味しくなかった = was not delicious). Add です after any form for politeness.
One important exception: いい (good) conjugates as よくない, よかった, よくなかった — it uses the older form よい as its base for all conjugations. い-adjectives are powerful because they conjugate independently, making them very flexible in sentence construction.

I-Adjective Conjugation Table

Form Suffix Example (Takai)
Present Affirmative
i + desu
Takai desu
Present Negative
ku + nai + desu
Takaku nai desu
Past Affirmative
katta + desu
Takakatta desu
Past Negative
ku + nakatta + desu
Takaku nakatta desu

Casual Forms

Form Suffix
Present Affirmative
i
Present Negative
kunai
Past Affirmative
katta
Past Negative
kunakatta

Meanings

I-adjectives are a class of Japanese adjectives that function as predicates, meaning they can end a sentence without a verb.

1

Present Affirmative

Describing a state in the present.

“{空|そら}が{青い|あおい}です。”

“{この|この} {本|ほん}は {面白い|おもしろい}です。”

2

Present Negative

Describing a state that is not true.

“{今日|きょう}は {暑く|あつく}ないです。”

“{この|この} {映画|えいが}は {面白く|おもしろく}ないです。”

3

Past Affirmative

Describing a state that was true in the past.

“{昨日|きのう}は {暑かった|あつかった}です。”

“{試験|しけん}は {難しかった|むずかしかった}です。”

Reference Table

Reference table for い-Adjectives: Japanese Adjectives That Conjugate
Form Structure Example
Present Affirmative
Stem + i + desu
{暑い|あつい}です
Present Negative
Stem + ku + nai + desu
{暑く|あつく}ないです
Past Affirmative
Stem + katta + desu
{暑かった|あつかった}です
Past Negative
Stem + ku + nakatta + desu
{暑く|あつく}なかったです
Te-form (connective)
Stem + ku + te
{暑くて|あつくて}
Adverbial
Stem + ku
{早く|はやく}

フォーマル度スペクトル

フォーマル
{美味しい|おいしい}です。

{美味しい|おいしい}です。 (Eating food)

ニュートラル
{美味しい|おいしい}です。

{美味しい|おいしい}です。 (Eating food)

カジュアル
{美味しい|おいしい}！

{美味しい|おいしい}！ (Eating food)

スラング
{うま|うま}！

{うま|うま}！ (Eating food)

Adjective Flow

Adjective

Present

  • i affirmative
  • kunai negative

Past

  • katta affirmative
  • kunakatta negative
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レベル別の例文

1

{今日|きょう}は {暑い|あつい}です。

2

{この|この} {映画|えいが}は {面白い|おもしろい}です。

3

{彼|かれ}は {優しい|やさしい}です。

4

{この|この} {パン|ぱん}は {美味しい|おいしい}です。

1

{昨日|きのう}は {寒く|さむく}なかったです。

2

{試験|しけん}は {難しかった|むずかしかった}です。

3

{部屋|へや}は {広く|ひろく}ないです。

4

{旅行|りょこう}は {楽しかった|たのしかった}です。

1

{彼|かれ}の {話|はなし}は {面白く|おもしろく}なかったです。

2

{昨日|きのう}の {パーティー|ぱーてぃー}は {賑やか|にぎやか}でした。

3

{この|この} {靴|くつ}は {安く|やすく}ないです。

4

{昨日|きのう}は {忙しかった|いそがしかった}です。

1

{そんなに|そんなに} {難しく|むずかしく}なかったですよ。

2

{もっと|もっと} {安く|やすく}してほしいです。

3

{昨日|きのう}より {暑く|あつく}なかったです。

4

{本当|ほんとう}に {嬉しかった|うれしかった}です。

1

{もっと|もっと} {早く|はやく} {来れば|くれば} {よかった|よかった}です。

2

{そんなに|そんなに} {面白く|おもしろく}もなかったです。

3

{昨日|きのう}の {会議|かいぎ}は {長かった|ながかった}です。

4

{もっと|もっと} {優しく|やさしく} {教えて|おしえて}ください。

1

{あまり|あまり} {美味しく|おいしく}はなかったのですが、{食べました|たべました}。

2

{昨日|きのう}の {夜|よる}は {静か|しずか}で {涼しかった|すずしかった}です。

3

{そんなに|そんなに} {難しく|むずかしく}は {ないはず|ないはず}です。

4

{もっと|もっと} {楽しく|たのしく} {過ごせ|すごせ}たはずです。

間違えやすい

い-Adjectives: Japanese Adjectives That Conjugate 対 Na-adjectives

Learners try to conjugate na-adjectives like i-adjectives.

い-Adjectives: Japanese Adjectives That Conjugate 対 Verbs

Learners treat adjectives as verbs.

い-Adjectives: Japanese Adjectives That Conjugate 対 Ii vs Yoi

Learners try to conjugate 'ii'.

よくある間違い

✗

Oishii-katta

✓

Oishikatta

Forgot to drop the 'i'.
✗

Oishii-na

✓

Oishii

Confusing with na-adjectives.
✗

Oishii-desu-katta

✓

Oishikatta desu

Wrong word order.
✗

Oishii-nai

✓

Oishikunai

Forgot to drop 'i' and add 'ku'.
✗

Oishikunai-katta

✓

Oishikunakatta

Double past tense error.
✗

Takai-deshita

✓

Takakatta desu

Using 'deshita' instead of conjugating the adjective.
✗

Oishii-desu-nai

✓

Oishikunai desu

Incorrect negation placement.
✗

Ii-katta

✓

Yokatta

Forgot the irregular stem for 'ii'.
✗

Takai-kunai

✓

Takakunai

Failed to drop 'i'.
✗

Oishikatta-deshita

✓

Oishikatta desu

Redundant past tense.
✗

Oishikunai-deshita

✓

Oishikunakatta desu

Failed to conjugate the negative form into the past.
✗

Takai-kute

✓

Takakute

Failed to drop 'i' for connective form.
✗

Oishikatta-nai

✓

Oishikunakatta

Incorrect past negative construction.

文型パターン

___ は ___ です。

___ は ___ くないです。

___ は ___ かったですか？

___ は ___ くなかったです。

Real World Usage

Texting constant

{暑い|あつい}！

Travel very common

{高い|たかい}ですか？

Food Reviews very common

{美味しい|おいしい}です。

Job Interview common

{忙しい|いそがしい}です。

Social Media constant

{楽しい|たのしい}！

Ordering Food common

{安い|やすい}のはどれですか？

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💡

The 'i' Rule

Always look for the 'i' at the end. If it's there, you're likely dealing with an i-adjective.
⚠️

Don't forget the 'desu'

In polite speech, always add 'desu' after the conjugated adjective.
🎯

Irregular 'ii'

Remember that 'ii' becomes 'yoi' when conjugating. This is the only major exception.
💬

Politeness matters

Using the casual form with strangers can sound rude. Stick to 'desu' until you know them well.

Smart Tips

Use 'kunakatta' instead of 'kunai deshita'.

Oishikunai deshita → Oishikunakatta desu

Immediately think 'yoi' for conjugation.

Ii-katta → Yokatta

Use the 'ku-te' form.

Takai, oishii → Takakute oishii

Always include 'desu'.

Takai → Takai desu

発音

o-i-shi-i

Long vowels

Ensure the 'i' sound is held for the correct length.

Statement

Takai desu ↘

Falling intonation for facts.

暗記しよう

記憶術

Think of the 'i' as a little person who gets kicked out (dropped) whenever you want to change the time or polarity.

視覚的連想

Imagine a letter 'i' standing on a stage. When you want to make it past tense, you push the 'i' off the stage and a big 'katta' sign falls in its place.

Rhyme

Drop the i, add the ku, past tense katta, that's what you do!

Story

Once there was an 'i' who loved to be present. But when it wanted to be negative, it called its friend 'kunai'. When it wanted to be in the past, it called its friend 'katta'. They always replaced the 'i' so it could rest.

Word Web

{暑い|あつい}{寒い|さむい}{高い|たかい}{安い|やすい}{美味しい|おいしい}{難しい|むずかしい}

チャレンジ

Write 4 sentences describing your day using one i-adjective in each of the 4 forms.

文化メモ

Adjectives are often used to express empathy.

They might use 'oishii' differently or use 'umai' more often.

Avoid overly casual adjectives.

I-adjectives evolved from ancient Japanese verbal forms that functioned as predicates.

会話のきっかけ

“

{今日|きょう}は {暑い|あつい}ですか？

“

{この|この} {料理|りょうり}は {美味しい|おいしい}ですか？

“

{昨日|きのう}の {映画|えいが}は {面白かった|おもしろかった}ですか？

“

{最近|さいきん} {忙しい|いそがしい}ですか？

日記のテーマ

Describe the weather today.
Describe a trip you took.
Describe a movie you didn't like.
Reflect on a busy week.

よくある間違い

Incorrect

正解

Incorrect

正解

Incorrect

正解

Incorrect

正解

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Test Yourself

Conjugate 'Takai' for past affirmative.

Kino wa ___ desu.

✓ Correct! ✗ Not quite. Correct answer: Takakatta
Past affirmative is 'katta'.
Which is the correct negative form of 'Oishii'? 選択問題

Oishii -> ?

✓ Correct! ✗ Not quite. Correct answer: Oishikunai
Drop 'i' and add 'kunai'.
Fix the sentence. Error Correction

Find and fix the mistake:

Kino wa samui-katta desu.

✓ Correct! ✗ Not quite. Correct answer: Samukatta desu
Drop 'i' before 'katta'.
Change to past negative. Sentence Transformation

Takai desu -> ?

✓ Correct! ✗ Not quite. Correct answer: Takakunakatta desu
Past negative is 'kunakatta'.
Is 'Shizuka' an i-adjective? True False Rule

True or False?

✓ Correct! ✗ Not quite. Correct answer: False
It's a na-adjective.
Complete the response. Dialogue Completion

A: Oishii desu ka? B: Iie, ___.

✓ Correct! ✗ Not quite. Correct answer: Oishikunai desu
Negative present.
Build a sentence. Sentence Building

Kino / samui / past negative.

✓ Correct! ✗ Not quite. Correct answer: Kino wa samukunakatta desu
Correct structure.
Match the form. Match Pairs

Match each item on the left with its pair on the right:

✓ Correct! ✗ Not quite. Correct answer: Past
Katta is past.

Score: /8

練習問題

8 exercises
Conjugate 'Takai' for past affirmative.

Kino wa ___ desu.

✓ Correct! ✗ Not quite. Correct answer: Takakatta
Past affirmative is 'katta'.
Which is the correct negative form of 'Oishii'? 選択問題

Oishii -> ?

✓ Correct! ✗ Not quite. Correct answer: Oishikunai
Drop 'i' and add 'kunai'.
Fix the sentence. Error Correction

Find and fix the mistake:

Kino wa samui-katta desu.

✓ Correct! ✗ Not quite. Correct answer: Samukatta desu
Drop 'i' before 'katta'.
Change to past negative. Sentence Transformation

Takai desu -> ?

✓ Correct! ✗ Not quite. Correct answer: Takakunakatta desu
Past negative is 'kunakatta'.
Is 'Shizuka' an i-adjective? True False Rule

True or False?

✓ Correct! ✗ Not quite. Correct answer: False
It's a na-adjective.
Complete the response. Dialogue Completion

A: Oishii desu ka? B: Iie, ___.

✓ Correct! ✗ Not quite. Correct answer: Oishikunai desu
Negative present.
Build a sentence. Sentence Building

Kino / samui / past negative.

✓ Correct! ✗ Not quite. Correct answer: Kino wa samukunakatta desu
Correct structure.
Match the form. Match Pairs

Takakatta -> ?

✓ Correct! ✗ Not quite. Correct answer: Past
Katta is past.

Score: /8

よくある質問 (8)

If it ends in 'i', it's almost certainly an i-adjective.

No, 'desu' is for polite speech. Use it with the dictionary form.

It's irregular. Use 'yoi' for all conjugations.

Very few. Most i-adjectives follow the rule perfectly.

Yes, use the 'ku' form.

'Kunai' is for adjectives; 'nai' is for verbs.

Yes, it conjugates like any other i-adjective.

It's part of the stem-suffix conjugation system.

Scaffolded Practice

1

1

2

2

3

3

4

4

Mastery Progress

Needs Practice

Improving

Strong

Mastered

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In Other Languages

Spanish low

Adjetivos con ser/estar

Japanese adjectives conjugate; Spanish adjectives do not.

French low

Adjectifs avec être

French adjectives are static; Japanese adjectives are dynamic.

German low

Adjektive mit sein

German adjectives don't conjugate for tense.

Chinese low

形容词 (Xíngróngcí)

Chinese adjectives are invariant.

Arabic low

الصفات (Al-Sifat)

Arabic adjectives don't conjugate for tense.

English low

Adjectives with 'to be'

English adjectives are not verbs.

Learning Path

Prerequisites

Next
な-Adjectives: Japanese Adjectives That Need な

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