Japanese has exactly one genuinely irregular い-adjective, and it happens to be one of the most-used words in the language: "good." You will meet it as いい, and it looks like an ordinary い-adjective — but the moment you try to make it negative, past, or turn it into an adverb, it refuses to follow the rules. The good news is that the irregularity is a single, learnable switch, and English speakers have a perfect analogy for it already.
Two dictionary forms: いい and よい
"Good" has two present-tense forms that mean the same thing:
- いい — the everyday spoken form. This is what you hear and say in conversation.
- よい（良い） — the more formal, written, and slightly older-sounding form. Common in writing, set phrases, and careful speech.
今日は天気がいい。
kyō wa tenki ga ii
The weather's nice today.
品質はよいが、値段が高い。
hinshitsu wa yoi ga, nedan ga takai
The quality is good, but the price is high. (written / formal)
Both are the plain present affirmative. Add です for politeness exactly like any other い-adjective: いいです / よいです.
この方法でいいですか。
kono hōhō de ii desu ka
Is this way okay?
The rule: conjugate only on the よ- stem
Here is the whole irregularity in one sentence: the dictionary form may be いい, but every other form is built from よ-, never い-. The instant you conjugate, いい is thrown out and よ- takes over.
| Form | Correct (よ- stem) | Wrong (regularized) |
|---|---|---|
| present | いい / よい | — |
| negative | よくない | ×いくない |
| past | よかった | ×いかった |
| past negative | よくなかった | ×いくなかった |
| te-form | よくて | ×いくて |
| adverb | よく | ×いく |
Everything in the "correct" column is a completely regular い-adjective conjugation — it is just built on よ- instead of the いい you started with. There is nothing new to learn about how to conjugate; you only have to remember to switch stems first.
Why it breaks: いい is a frozen softening of よい
This is not random cruelty. Historically the word was よい, and よい softened in casual speech to いい — the same lazy-mouth smoothing you hear in fast English ("going to" → "gonna"). But the softening only froze in the dictionary form. The conjugated forms kept the older よ-. So よ- is not an exception you memorize in isolation; it is the original stem re-emerging the moment you leave the dictionary form behind. Picture いい as a thin modern shell over an older よ- core: touch it (conjugate it) and the shell falls away.
The negative: よくない / よくありません
私はあまり頭がよくないから、メモを取ります。
watashi wa amari atama ga yokunai kara, memo o torimasu
I don't have a great memory, so I take notes.
この椅子、座り心地がよくないね。
kono isu, suwarigokochi ga yokunai ne
This chair isn't comfortable to sit in, is it.
For polite negatives, both よくないです and よくありません work, just like any other い-adjective (note: 頭がいい "smart / good memory" and its negative 頭がよくない are extremely common, so drill this pair).
The past: よかった — and the idiom you will use daily
昨日の映画、すごくよかったよ。
kinō no eiga, sugoku yokatta yo
Yesterday's movie was really good.
温泉は本当に気持ちよかった。
onsen wa hontō ni kimochi yokatta
The hot spring felt really good.
よかった also lives a second life as a stand-alone idiom meaning roughly "oh good / thank goodness / I'm so glad / what a relief." It is one of the highest-frequency emotional responses in spoken Japanese, and you should recognize it instantly.
よかった、間に合った！
yokatta, ma ni atta
Oh good, we made it in time!
無事でよかったです。
buji de yokatta desu
I'm so glad you're safe / all right.
Compounds inherit the irregularity
Any adjective that ends in いい because it is built on "good" carries the same よ- switch. This is where learners who only memorized いい in isolation get caught, because these compounds are everywhere in natural speech.
昔の写真のお父さん、めっちゃかっこよかった。
mukashi no shashin no otōsan, meccha kakkoyokatta
Dad looked so cool in the old photos. (casual)
このソファ、座り心地が気持ちよくて、つい寝ちゃう。
kono sofa, suwarigokochi ga kimochi yokute, tsui nechau
This sofa is so comfy I end up dozing off.
明日なら都合がいいですが、今日は都合がよくないです。
ashita nara tsugō ga ii desu ga, kyō wa tsugō ga yokunai desu
Tomorrow works for me, but today isn't convenient.
So かっこいい → かっこよかった / かっこよくない, 気持ちいい → 気持ちよかった / 気持ちよくて, 都合がいい → 都合がよくない, 仲がいい → 仲がよかった. Same word, same rule, every time.
よく: an adverb you will use constantly
The adverb form よく (from the よ- stem, of course — never ×いく) is one of the most frequent adverbs in the language, and it carries two everyday meanings that beginners meet early: "well / properly" and "often." Context decides which.
先生の説明がよく分かりました。
sensei no setsumei ga yoku wakarimashita
I understood the teacher's explanation well.
子供の頃、この川でよく泳いだ。
kodomo no koro, kono kawa de yoku oyoida
When I was a kid, I often swam in this river.
Because よく is so common, saying ×いく分かる instead marks you as a beginner instantly — another reason the よ- reflex is worth over-drilling.
Common mistakes
❌ 私は頭がいくない。
Incorrect — 'good' never conjugates on い-; use the よ- stem.
✅ 私は頭がよくない。
watashi wa atama ga yokunai
I'm not that smart / my memory isn't good.
❌ 昨日の試合はいかった。
Incorrect — the past of いい is not ×いかった.
✅ 昨日の試合はよかった。
kinō no shiai wa yokatta
Yesterday's game was good.
❌ その髪型、超かっこいかったよ。
Incorrect — かっこいい inherits the irregularity too.
✅ その髪型、超かっこよかったよ。
sono kamigata, chō kakkoyokatta yo
That hairstyle looked super cool.
❌ 説明が下手で、いく分からなかった。
Incorrect — the adverb 'well' is よく, not ×いく.
✅ 説明が下手で、よく分からなかった。
setsumei ga heta de, yoku wakaranakatta
The explanation was poor, so I couldn't really understand.
Every one of these errors is the same slip: regularizing いい as if the dictionary form were the stem. The fix is a reflex you can build in a day — the moment "good" needs to change in any way, mentally swap いい for よ- first, then conjugate normally. Do that and 良い（よい）, よくない, よかった, よくなかった, よくて, and よく all come out right without any further thought.
Now practice Japanese
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Start learning Japanese→Related Topics
- i-Adjectives: PresentN5 — The dictionary form of an い-adjective ends in the kana い and works two ways with no helper word — straight before a noun (面白い本) and as a complete predicate ending a sentence (この本は面白い) — because the adjective already contains its own 'to be.'
- i-Adjectives: Negative (〜くない)N5 — To negate an い-adjective you drop the final い and add くない (高い→高くない); the polite versions are 〜くないです and 〜くありません — and crucially you never use じゃない, which belongs to nouns and な-adjectives.
- i-Adjectives: Past (〜かった)N5 — To put an い-adjective in the past you drop the final い and add かった (楽しい→楽しかった); the polite past is 〜かったです — never ×楽しいでした — because with a true い-adjective the word itself carries the tense, not the copula.
- Two Adjective ClassesN5 — Japanese has two structurally different kinds of adjective — い-adjectives that conjugate themselves like verbs, and な-adjectives that are really nouns borrowing the copula — and this single split explains every adjective form you will ever meet.