The いい / 良い Irregular: Why the Past Is よかった
The いい / 良い irregular is the single い-adjective in modern Japanese whose present-affirmative form does not inflect. The present is いい (or its more formal twin 良い / よい), but every other cell of the paradigm switches to the stem よ before the regular suffixes attach.123 For an N5 learner who has just internalised the clean 大きい / 大きくない / 大きかった pattern, this one lemma has to be memorised whole. It is also the lemma that produces よかった, one of the most-heard utterances in spoken Japanese.14
Overview
Where this article sits in the adjectives map
Modern Japanese has many regular い-adjectives that inflect from a single visible stem (大きい → 大きくない → 大きかった), and exactly one lemma that does not: 良い. Its present-affirmative is the colloquial いい, but its non-present cells inflect from a different stem, よ.1235 This article covers only the swap itself and its consequences. The regular paradigm that 良い's non-present cells still follow after the stem has switched is treated as background.16
This framing follows standard textbook practice. Both Genki I and Minna no Nihongo I introduce いい alongside the regular paradigm and flag it as the only irregular adjective in the language.16 The qualifier matters: いい is the only standardly recognised irregular い-adjective. The rest of this article unpacks the one swap that earns it that label.
The one-line rule
The present-affirmative is the only cell where いい appears in standard Japanese; every other inflected form (negative, past, past-negative, te-form, adverbial, conditional, and their polite layers) uses the stem よ plus the regular suffix.7829
Wiktionary states the same constraint plainly: いい "does not inflect in Standard Japanese" and "is only used in the 終止形 (terminal form) and 連体形 (attributive form)."8 Everything else supplets from よい / 良い: the past is よかった, the negative is よくない, the te-form is よくて, the adverbial is よく.72910
Why this is the only irregular い-adjective
The explanation is historical, not synchronic (about the present-day system alone). いい is a colloquial softening of older よい, which is still current in writing and formal speech. The older form retained its full inflectional paradigm, the colloquial form did not, and speakers continue to use the older paradigm whenever they need anything other than the bare present-affirmative.7823
The rest of this article unpacks that trail and its effects: the two spellings, the eight-cell conjugation grid, the four impossible forms learners coin by analogy, the classical 良し → 良い → いい etymology, and the open class of compound adjectives that inherit the same irregularity through their いい tail.
いい vs. 良い: Two Spellings of One Word
The three written shapes: いい, よい, 良い
The present-affirmative of this lemma is written three ways in modern Japanese: kana いい (more colloquial), kana よい (more formal), and kanji 良い (read either いい or よい depending on register and reader).1147
The two major monolingual dictionaries treat them as one word with register variants, not three separate entries. 大辞林 lists 良い with two readings (いい・よい) under one head entry, and 日本国語大辞典 follows the same convention.114 Wiktionary's English entry explicitly notes that "いい can appear as 良(よ)い" and that the reading よい "sounds more formal/old-fashioned" while carrying the same meaning.910
これはいい本です。10
"This is a good book."
これが良い。4
"This is good."
The two sentences say the same thing; only the register and spelling differ. The kanji 良い allows either reading, so a reader of the second sentence might voice it as either よい or いい depending on context.
Which one to say out loud
In everyday spoken Japanese the present-affirmative is overwhelmingly いい; よい in casual conversation sounds stiff, older, or regional.392 Wiktionary's gloss on いい marks it as "good (colloquial / informal usage)" and treats よい as the more formal reading of the same lemma.87
天気がいいですね。1
"The weather is nice, isn't it?"
良い一日をお過ごしください。4
"Have a good day."
The casual remark about the weather fits the いい reading. The set greeting about a good day conventionally takes よい. Both readings of 良い are valid; the choice tracks register.
Where 良い is the safer choice in writing
Dictionaries lemmatise, or list as the main dictionary form, the kanji form 良い rather than the colloquial いい. 大辞林 and 日本国語大辞典 both list 良い as the head entry with いい・よい as the readings.114 News prose, business email, and dictionary entries default to 良い in the present-affirmative, more often voiced as よい than as いい inside those genres.94
This is editorial convention, not a grammatical rule. A learner can write either spelling and be understood. The choice signals register.
The Full Conjugation Paradigm
The eight core cells (plain × polite × present × past × affirmative × negative)
Applied to this lemma, the eight standard cells of an い-adjective predicate produce the plain × polite × present / past × affirmative / negative grid below.16210 Genki I Chapter 5 teaches the polite quadrant of this grid (いいです / よくありません / よかったです / よくありませんでした) as a single block. It labels the lemma "the only irregularly conjugated adjective."112
| Tense / polarity | Plain | Polite |
|---|---|---|
| Present-affirmative | いい (or 良い / よい) | いいです (or 良いです / よいです) |
| Present-negative | よくない | よくないです / よくありません |
| Past-affirmative | よかった | よかったです |
| Past-negative | よくなかった | よくなかったです / よくありませんでした |
The shape of the table is the point. Every cell except the two affirmative present cells uses the stem よ. The present-affirmative is the only slot where いい (or its more formal twin よい) appears.7829
The diagram captures the one decision the learner has to make whenever the form changes: the present-affirmative stays いい. Everything else routes through the よ stem.
The present is the only いい cell
The constraint is morphological (about word form) and absolute in standard Japanese: いい does not take any inflectional suffix directly. Whenever an inflectional suffix attaches, the stem switches to よ first.823 Rephrased from Wiktionary: いい "is only used in the 終止形 (terminal form) and 連体形 (attributive form)"; everything else supplets from よい.87
No other regular い-adjective behaves this way. The lemma's whole irregularity is contained in that one stem swap.
Past-affirmative: よかった / よかったです
The past-affirmative is よかった (plain) and よかったです (polite). The past-tense suffix かった attaches in the regular way, but to the stem よ, not い.1210
昨日の天気はよかったです。10
"The weather yesterday was good."
Standalone よかった is also one of the highest-frequency reactive utterances in spoken Japanese. It is used for relief ("thank goodness"), congratulation ("I'm glad for you"), and acknowledgement of good news; 日本国語大辞典 records this as a discourse extension of the past-tense adjectival form.411
試験に受かって、よかった。1
"I passed the exam, and I'm relieved."
Present-negative: よくない / よくないです / よくありません
The present-negative is よくない (plain), with two polite variants: よくないです (colloquial polite) and よくありません (formal polite). The stem swaps to よ first, then takes the regular くない suffix.16210 Genki I teaches the formal polite form よくありません as part of the Chapter 5 irregular block.112
その映画はよくないです。10
"That movie is not good."
体によくありません。1
"It is not good for the body."
The polite-layer split (くないです vs. くありません) is the regular い-adjective polite split. The irregular lemma does not affect it. The same colloquial-polite vs. formal-polite choice applies to every other い-adjective in the language.
Past-negative: よくなかった / よくなかったです / よくありませんでした
The past-negative is よくなかった (plain), with the same colloquial-polite and formal-polite split as the present-negative: よくなかったです and よくありませんでした.162 Once the stem has switched, the shape is fully predictable: stem よ + adverbial く + regular ない / なかった / ありません / ありませんでした.293
昨日の試合はよくなかったです。6
"Yesterday's match was not good."
Te-form: よくて
The te-form of this lemma is よくて. It is formed by attaching the い-adjective て-suffix くて to the よ stem; いくて is not a form of this adjective in standard Japanese.2910
天気がよくて、散歩した。10
"The weather was nice, so I took a walk."
The te-form on this lemma does the same connecting and causal jobs it does for any other い-adjective; only the stem changes.
Adverbial form: よく
The adverbial form of this lemma is よく; いく is not the adverbial of いい. (いく is the adverbial of 行く, a homophone that learners regularly confuse.)3119 大辞林 lists よく as a high-frequency adverb with two distinct dictionary senses: a manner reading ("well, skilfully, thoroughly") and a frequency reading ("often, frequently"). Both senses derive historically from the adverbial of 良し but are now selected by context.114
彼はよく勉強する。11
"He studies often."
よく聞いてください。11
"Please listen carefully."
The two senses share one shape. Context, not form, selects which reading is intended.
The Four Forms You Will Try to Coin (And Why They Are Wrong)
All four errors share one root cause: applying the regular い-adjective paradigm directly to the colloquial present stem いい instead of switching to the よ stem before the suffix attaches.1329 The corrections are uniform, and the diagnosis is the same in every case.
The dotted paths are the ones learners try. The solid paths are the standard forms. Every wrong form vanishes if the stem swap happens first.
Wiktionary attests いかった as a non-standard past-affirmative in Niigata and いがった in Tōhoku.8 Outside those regional dialects, the analogy-based forms in this section are production errors. A learner aiming for standard Japanese should treat them as wrong.
いくない → よくない
This is the most common beginner regularisation. Standard Japanese rejects いくない because the negative suffix cannot attach to the colloquial present stem いい. The stem switches to よ first, then takes くない.1329
この本はよくないです。10
"This book is not good."
いかった → よかった
This is the second most common error. The past-tense suffix かった attaches to the よ stem, not the いい stem. The regularised いかった survives only as a Niigata dialect form.821
昨日の天気はよかったです。10
"Yesterday's weather was good."
いくありません → よくありません
This is a layered regularisation: the polite-formal suffix ありません attaches to the also non-standard adverbial いく. The correct form attaches ありません to the adverbial よく, which itself sits on the よ stem.1132
その映画はよくありません。1
"That movie is not good."
いくなかった → よくなかった
This is the past-negative version of the same error. The negative なかった must attach to よく, which sits on the よ stem.132
昨日の試合はよくなかったです。6
"Yesterday's match was not good."
Why the Irregularity Exists: The 良し → 良い → いい Trail
The classical adjective was 良し (yoshi)
Classical Japanese had a ク-adjective 良し with the inflection set 良く (連用形) / 良し (終止形) / 良き (連体形) / 良けれ (已然形), all built from the stem よ. The terminal form 良し ended in -し, and the noun-modifying form 良き ended in -き. These were two morphologically distinct shapes.141511 Wiktionary records the same paradigm, deriving modern よい from "Old Japanese adjective yoshi" via the terminal-form route /joɕi/ → /joi/.7
The terminal form lost its -し and became よい
A Late Old / Early Middle Japanese sound change merged the classical -し (shūshikei) and -き (rentaikei) endings of ク-adjectives into a single -い form. This produced the modern い-adjective ending across the language. 良し / 良き collapsed to 良い by the same shift that produced 高い from 高し / 高き.14152
After this merger, よい was the present-affirmative; the rest of the paradigm continued to inflect from the stem よ in the regular い-adjective way (よかった, よくない, よくて, よく).1423
Colloquial speech then softened よい to いい
In spoken Japanese the present-affirmative よい underwent a further phonological shift to いい. Wiktionary reconstructs the sound-change chain /joɕi/ → /joi/ → */jiː/ → /iː/. The colloquial bi-moraic いい emerges as a present-only form.87
The softening applied only to the terminal / attributive slot of the paradigm. The rest of the cells had already become fixed from the stem よ and stayed there. That is why the modern split between present-only いい and よ-stem everything else looks the way it does.872
The diagram makes the split visible: the bottom branch (every non-present cell) became fixed from よい before the top branch (present-affirmative) softened to いい. The colloquial softening only ever reached the one slot.
Why no other い-adjective drifted this way
良い is one of the highest-frequency adjectives in the language. High-frequency words are the ones that accumulate enough conversational wear to spawn shortened or softened colloquial variants. Lower-frequency items keep their phonological shape intact.4316
The same frequency-driven irregularisation pattern is documented for the highest-frequency verbs: する and 来る are likewise irregular and likewise hyper-frequent.145 良い is the single い-adjective that reached the frequency threshold where a colloquial present-only variant could become stable alongside the full paradigm.
Compound Adjectives Inherit the Irregularity
The compound rule in one line
Any compound adjective whose tail is いい inherits the full irregularity from that tail. Conjugation happens on the いい tail only; the head morpheme is left unchanged.17910 Wiktionary's entry for 格好いい confirms the inheritance by listing the conjugated forms as 格好よかった, 格好よくて, 格好よくない, 格好よく, all with the よ-stem swap on the いい tail.17
Worked examples
The four compounds below are among the highest-frequency cases an N5 learner meets. Each shows the past-affirmative and past-negative; the rest of the paradigm follows by the same rule.
| Compound | Past-affirmative | Past-negative |
|---|---|---|
| 格好いい ("cool, stylish") | 格好よかった | 格好よくなかった |
| 仲がいい ("on good terms") | 仲がよかった | 仲がよくなかった |
| 気持ちいい ("feels good, pleasant") | 気持ちよかった | 気持ちよくなかった |
| 都合がいい ("convenient") | 都合がよかった | 都合がよくなかった |
The pattern is mechanical. Each tail いい becomes よ + the regular suffix. Each head (格好, 仲が, 気持ち, 都合が) stays unchanged.17911
昨日のパーティーは格好よかった。17
"Yesterday's party was cool."
海で泳いで、気持ちよかった。9
"I swam in the sea and it felt great."
Why this matters more than it sounds
Compound adjectives whose tail is いい form an open class in modern Japanese: new compounds enter the lexicon (e.g., かっこいい is itself a 20th-century formation, first cited 1963), and all of them inherit the irregularity by virtue of their tail.179
For learners, the compound form is often the first place they meet a よ form in real use. Compounds like かっこいい, 気持ちいい, and 都合がいい appear in N5-level conversation well before the bare adjective surfaces in formal writing.910
Nuance and Register
When 良い beats いい in writing
Dictionaries lemmatise 良い (with readings いい・よい) rather than the colloquial いい; 大辞林 and 日本国語大辞典 both follow this convention.114 Set phrases of the よい一日を type ("have a good day"), 体調が良い ("in good health"), and 〜より良い ("better than") prefer the 良 / よ shape in writing, even though the same meaning in casual speech prefers いい.911
良い週末を。4
"Have a good weekend."
The set phrase is conventional in the よい reading; いい would sound less idiomatic in this frame.
When いい beats よい in speech
Spoken Japanese strongly prefers いい in the present-affirmative; よい in casual speech can sound stiff, older, or regional.392
The polite layer (いいです / いいですよ / いいですね) is normal in everyday conversation regardless of speaker age or formality of the situation. The です bolt-on is itself the politeness marker; switching from いい to よい inside conversation is not how speakers signal politeness.93
Frozen expressions that lock in one shape
A handful of expressions only take one form. Read them as set phrases rather than test cases for the rule.
よろしい is a polite-register cousin of いい / よい, a separate lexeme (dictionary word) historically derived from the same stem family. It conjugates regularly (よろしくない / よろしかった / よろしくて) and does not show the irregular swap, because the irregularity is a property of the lemma 良い, not of the stem よ in general.11184
よろしく ("regards, please be kind to me") is the adverbial of よろしい, frozen into a discourse formula. Its base よろしい derives etymologically from the same よろ- root family as よい / いい but functions as its own lexical item.184
どうぞよろしくお願いします。18
"Pleased to meet you."
もういい ("that's enough") and どうでもいい ("doesn't matter") lock the present-affirmative shape いい into a frozen idiomatic frame. These phrases do conjugate (もうよかった is valid when the predicate is past), but they are most often encountered in the present.811
Good to know
The polite-form trap: いいです is fine, but いいでした is not
Learners who treat いいです as a noun + copula sometimes try いいでした for the past. The correct past-polite is よかったです, not いいでした. The reason is that です on an い-adjective is a register bolt-on, not a copula carrying tense. The past has to live on the adjective stem itself, and for this lemma the stem switches to よ before かった attaches.1313
昨日はよかったです。1
"Yesterday was good."
The same rule governs every い-adjective. 大きい becomes 大きかったです, never 大きいでした. The irregular lemma is special in its stem, not in its polite-layer behaviour.
Why よく has two unrelated jobs
The adverb よく means both "well" (manner: よく書ける, "writes well") and "often" (frequency: よく行く, "goes often"). 大辞林 lists these as two senses of the same adverb, both deriving historically from the adverbial of 良し. It is the same form, with two readings selected by context.114 Both are part of the irregular paradigm, and neither surfaces as いく.
The exclamation よかった is a culture point, not just a form
Standalone よかった is one of the highest-frequency reactive utterances in conversation. It is used for relief ("thank goodness"), congratulation ("I'm glad for you"), and acknowledgement of good news. 日本国語大辞典 records this as a discourse extension of the past-affirmative.411 A learner should recognise よかった as a stock expression of relief, not only as a conjugated past form.
Where the rule stops
The irregular paradigm covers いい, 良い, よい, and every compound adjective whose tail is いい. It does not extend to adjectives whose final い is the regular inflectional ending of a different stem. 大きい, for example, has the past 大きかった, not 大よかった. The い of 大きい is the inflectional ending of a regular i-adjective, not the いい lemma.295
The diagnostic is the lemma, not the surface shape. If the adjective in question is 良い (or a compound whose tail is いい), the swap applies. Otherwise, the regular paradigm applies unchanged.
See also
- な-Adjective Conjugation in Japanese: All Tenses and Forms
- Adjective Stem Nominalization in Japanese: ～さ vs. ～み
- Polite vs. Plain Japanese: です/ます vs. だ (丁寧体・普通体)
- The Ta-Form in Japanese: Construction Rules