いい (Irregular Adjective)いい / 良い
いい (good) is the ONLY irregular い-adjective in Japanese. Its dictionary form is いい, but ALL conjugated forms use the older stem よ- (from 良い yoi). This means the negative is よくない (NOT いくない), the past is よかった (NOT いかった), and so on. This is the single most common adjective conjugation mistake among learners.
いい (good) is the ONLY irregular い-adjective in Japanese. Its dictionary form is いい, but ALL conjugated forms use the older stem よ- (from 良い yoi). This means the negative is よくない (NOT いくない), the past is よかった (NOT いかった), and so on. This is the single most common adjective conjugation mistake among learners.
Why is いい irregular?
Historically, the adjective was 良い (yoi), and all conjugation was built from the よ- stem: よくない, よかった, よくて, etc. Over time, the dictionary form shifted from よい to いい in spoken Japanese, but the conjugated forms stayed anchored to the original よ- stem. So modern Japanese has a split: the citation form is いい, but everything else uses よ-.
Complete conjugation of いい
The rule is simple: only the dictionary form uses いい. Every other form uses よ- and then follows standard い-adjective rules (drop い, add suffix).
All forms
Non-past: いい / 良い (ii / yoi). Negative: よくない (yokunai). Past: よかった (yokatta). Past negative: よくなかった (yokunakatta). て-form: よくて (yokute). Adverbial: よく (yoku). Conditional: よければ (yokereba).
Compounds with いい
When いい appears in compounds, the same rule applies. かっこいい (cool) → かっこよくない, かっこよかった. 頭がいい (smart) → 頭がよくない, 頭がよかった. 仲がいい (get along well) → 仲がよくない, 仲がよかった.
よい vs いい — when to use which
In conversation, いい is standard. In formal writing, news articles, and official documents, 良い (yoi) is preferred. Both are correct in the dictionary form. However, in conjugated forms, both spoken and written Japanese use the よ- stem: よくない, よかった. You will never see いくない or いかった in any register.
いい conjugation vs standard い-adjective
| Form | いい (irregular) | 高い (regular) | What NOT to do |
|---|---|---|---|
| Non-past | いい / 良い | 高い | — |
| Negative | よくない | 高くない | いくない ✗ |
| Past | よかった | 高かった | いかった ✗ |
| Past negative | よくなかった | 高くなかった | いくなかった ✗ |
| て-form | よくて | 高くて | いくて ✗ |
| Adverbial | よく | 高く | いく ✗ |
Example sentences
この本はとてもいい。
Kono hon wa totemo ii.
This book is very good.
Dictionary form: いい
天気がよくない。
Tenki ga yokunai.
The weather is not good.
Negative: よくない (NOT いくない)
昨日の映画はよかった。
Kinou no eiga wa yokatta.
Yesterday’s movie was good.
Past: よかった (NOT いかった)
成績がよくなかった。
Seiseki ga yokunakatta.
My grades were not good.
Past negative: よくなかった
天気がよくて散歩した。
Tenki ga yokute sanpo shita.
The weather was nice so I took a walk.
て-form: よくて
かっこよくなりたい。
Kakko yoku naritai.
I want to become cool.
Compound: かっこいい → かっこよく + なる
Common mistakes
いくない (ikunai)
よくない (yokunai)
All conjugated forms of いい use the よ- stem. いくない does not exist in Japanese.
いかった (ikatta)
よかった (yokatta)
The past of いい is よかった, from the original stem 良(よ). いかった is never correct.
かっこいくない
かっこよくない
Compounds with いい follow the same irregular pattern. The いい portion always becomes よ- when conjugated.
いいだった
よかった
Double error: い-adjectives never use だった, and いい uses よ- for past. The correct past is simply よかった.
Frequently asked questions
Is いい the only irregular い-adjective?
Yes. Every other い-adjective in Japanese conjugates by the regular pattern (drop い, add suffix). いい is unique because its dictionary form diverged from its conjugation stem centuries ago.
Can I use 良い (yoi) instead of いい in conversation?
You can, but it sounds slightly formal or literary. In everyday speech, いい is the natural choice for the non-past affirmative. For all other forms, both spoken and written Japanese use the よ- stem anyway.
How do I conjugate かっこいい (cool)?
Treat the いい part as irregular: かっこいい → かっこよくない → かっこよかった → かっこよくなかった → かっこよくて → かっこよく. The かっこ part stays fixed.
What is the negative of いい?