Japanese Adjectives Overview: The Two Classes (い-形容詞 vs な-形容詞)
Japanese adjectives split into two productive classes. い-形容詞 inflect themselves the way verbs do. な-形容詞 stay fixed and let the copula carry the inflection.12 A beginner who sees きれい ending in い and treats it like 大きい has taken the most common wrong turn the system permits. This article aims to give you the correct mental model on the first pass.34 The sections below cover how to identify a class on sight, what each class does, the きれい trap, and where to go next for the actual conjugation tables.
Overview
The two-class split is the organising principle of Japanese adjectives. First, you need the class names, where adjectives sit in a sentence, and what this article does and does not teach. Everything else in the article depends on that orientation.
Why Japanese has two adjective classes, not one
Modern school grammar (学校文法) recognises two productive adjective classes among the 10 hinshi parts of speech, or word classes. The first is 形容詞 (keiyōshi), conventionally called い-adjectives because the dictionary form ends in the kana い. The second is 形容動詞 (keiyōdōshi, literally "adjective-verb"), conventionally called な-adjectives because they take な before a head noun.321
形容詞 come from Old Japanese as a fully inflecting word class. In Old Japanese they split into two sub-classes whose stems took the suffix 〜く (-ku) or 〜しく (-shiku); the two sub-classes merged in Late Middle Japanese (roughly 1200 to 1600) to yield the single modern paradigm, but the historical -shiku class is still visible in the modern -しい group (悲しい, 楽しい, 嬉しい).56
形容動詞 are common in Sino-Japanese (漢語) vocabulary (静か, 有名, 自由, 安全, 必要), plus a smaller but productive set of katakana borrowings (ハンサム, スマート, モダン). They entered the language as nouns or noun-like elements and used the copula as their inflectional engine. Native Yamato な-adjectives also exist (静か, 豊か, 穏やか), typically carrying the derivational suffixes 〜か or 〜らか.347
The term 形容動詞 itself was coined for this class by the grammarian Haga Yaichi (芳賀矢一) in the early twentieth century; before then, the class had no unified school-grammar label.783
The two classes differ in how open they are to new words. Native speakers do not coin new い-adjectives; the inventory is essentially the one inherited from Old Japanese. Every productive loanword adjective entering the language joins the な-class instead, which is why フレッシュ and スタイリッシュ both pattern with な.96
The practical takeaway is that the two classes spend their grammar differently: one inflects in its own stem, while the other borrows inflection from the copula. The rest of the article unpacks that difference.
Where adjectives sit in a Japanese sentence
Adjectives in Japanese do two grammatical jobs. The first is attributive use: modifying a following noun, as in 大きい本 ("a big book") or 静かな部屋 ("a quiet room"). The second is predicative use: standing at the end of a sentence, as in この本は大きい ("this book is big") or この部屋は静かだ ("this room is quiet").110112
い-形容詞 use the same surface form for both jobs. The form does not change between the attributive slot and the predicative slot. This comes from a Late Middle Japanese sound change that reduced the older attributive 〜き to 〜い.56
大きい本がある。10
"There is a big book."
この本は大きい。10
"This book is big."
な-形容詞 surface in two different shapes depending on the slot: な before a head noun and だ at the sentence end. The な and the だ are forms of the same copula, not two unrelated particles, and that is the school-grammar reason 形容動詞 carries the name "adjective-verb."3127
静かな部屋がいい。11
"A quiet room would be good."
この部屋は静かだ。11
"This room is quiet."
Both classes follow the broader Japanese head-final, subject-object-verb (SOV) orientation: the adjective comes before its head noun in attributive use, and the predicate occupies the sentence-final slot.913 A planned sibling article will handle the deeper attributive-versus-predicative analysis. Here, it is treated only as a concept.
Audience and what this article does not cover
This is a first-contact article for N5 learners. It gives the two-class map, the identification routine, and where to go next.1415 Reading-level kanji are glossed where they could trip a learner; the Japanese examples themselves stay at their natural register.
Out of scope here:
- The full conjugation paradigm for either class (い-形容詞 and な-形容詞).
- The いい / 良い irregular treatment.
- The attributive-versus-predicative deep dive.
- The adverbial く / に formation.
- Comparison patterns (より, の方が, 一番) and equality patterns (と同じ, ぐらい, ほど).
- The full な-adjective-vs-noun blurred-boundary debate.
Each topic lives, or will live, in a dedicated article in the same subcategory.
The two classes: い-形容詞 and な-形容詞
The clearest way to define the two classes is by what each one conjugates like. い-形容詞 conjugate like verbs (the adjective itself inflects); な-形容詞 conjugate like nouns with a copula attached (the stem is inert; the copula carries everything).
い-形容詞 (i-adjectives): conjugate like verbs
い-形容詞 are fully inflecting predicates. They do not need a copula in plain register; the adjective itself carries tense and polarity.1102 They use the same six-cell paradigm (未然形, 連用形, 終止形, 連体形, 仮定形, 命令形) that organises verb conjugation. That is the structural reason the school-grammar name 形容詞 places the class alongside verbs.652
The stem, or the part before the final い, combines with all suffixes:
| Form | 大きい (canonical) | What is added |
|---|---|---|
| Dictionary / non-past | 大きい | 大き + い |
| Past | 大きかった | 大き + かった |
| Negative | 大きくない | 大き + くない |
| te-form (linking) | 大きくて | 大き + くて |
| Adverbial | 大きく | 大き + く |
Historically, the inflected forms come from the kari-conjugation (カリ活用), a contraction of the continuative 〜く plus the existential verb あり: 寒く + あり → 寒かり → 寒かっ-. That is why every past, negative, and conditional form of a modern い-形容詞 contains a -k- or -kar- residue.65
今日は寒い。10
"It's cold today."
昨日は寒かった。10
"It was cold yesterday."
In polite register, です attaches directly to the inflected plain form without changing the adjective itself: 大きいです, 大きかったです, 大きくないです, 大きくなかったです. Here, です is a politeness wrapper, not the copula doing inflectional work. Its only job is to add politeness to the utterance.1616
な-形容詞 (na-adjectives): conjugate like nouns + な
The な-形容詞 stem does not inflect. The following copula carries all tense and polarity marking.110112
| Form | 静か (canonical) | What is added |
|---|---|---|
| Attributive (+ noun) | 静かな部屋 | stem + な + N |
| Predicate non-past | 静かだ | stem + だ |
| Predicate past | 静かだった | stem + だった |
| Predicate negative | 静かじゃない | stem + じゃない |
| te-form (linking) | 静かで | stem + で |
な is the attributive (連体形) form of the copula; だ is the predicative (終止形) form of the same copula. Historically, な is a contraction of the classical attributive copula なる, itself from earlier 〜に + ある "to be in [such a state]," preserved in modern Japanese specifically in this attributive slot.7176 The te-form で is the continuative form of the same copula, which is why 静かで can link a な-形容詞 to a following predicate.11011
この部屋は静かだ。11
"This room is quiet."
昨日は暇じゃなかった。11
"I wasn't free yesterday."
The same word takes な before a noun and だ at the sentence end because な and だ are forms of the same copula. This is also why a な-形容詞 stem alone is not a complete plain-register sentence; the copula must be supplied.1126
In plain-register Japanese, remember that the copula is making the predicate, not the stem.112
- Incorrect: 静か。 (intended as "It's quiet.")
- Correct: 静かだ。 / 静かです。
Casual speech does allow a dropped copula in rising-intonation questions such as 元気? ("doing OK?"), but that is the copula being elided, not the stem standing alone as a predicate.1718
This construction explains the school-grammar name 形容動詞 ("adjective-verb"): the copula gives the whole predicate a verb-like inflectional paradigm, even though the adjective stem itself does not move.32
Side-by-side: how the same job gets done by each class
The two classes can be summarised by listing five inflectional jobs and showing each class doing them with a different morpheme, or meaningful word part.110112
| Job | い-形容詞 (大きい) | な-形容詞 (静か) |
|---|---|---|
| Attributive (+ noun) | 大きい本 ōkii hon | 静かな部屋 shizuka na heya |
| Predicate non-past | 本は大きい ōkii | 部屋は静かだ shizuka da |
| Predicate past | 大きかった ōkikatta | 静かだった shizuka datta |
| Predicate negative | 大きくない ōkikunai | 静かじゃない shizuka janai |
| te-form (linking) | 大きくて ōkikute | 静かで shizuka de |
Two short examples show the rows in motion. The first chains an い-form to a な-form; the second goes the other way.
大きくて静かな家を探している。10
"I'm looking for a big, quiet house."
静かで広い部屋がいい。11
"A quiet, spacious room would be good."
How to tell which class a word belongs to
An adjective's class is not always obvious from its dictionary form. A reliable approach is to start with a quick rule of thumb, notice where it fails, and fall back on two tests that work in every environment.
The default test: does it end in い?
For the overwhelming majority of common adjectives, the dictionary form of an い-形容詞 ends in the kana い, preceded by another kana (寒い, 高い, 楽しい, 新しい). な-形容詞 dictionary forms end in any of the other kana, including あ (静か, 元気, 有名, ハンサム).110112
This is a first pass, not a true test. It fails on a small set of common な-形容詞 whose dictionary form happens to end in い; the next subsection lists them. It also fails when an い-final sound is locked inside a kanji rather than being a grammatical い.346 When you need certainty rather than a rule of thumb, the two tests two subsections down decide the class in any environment.
The kirei trap and the small list of い-final な-adjectives
A short, conservative list of common な-形容詞 happens to end in the kana い and trips the rule of thumb. The list as it surfaces across major learner references and JMdict includes:34619
- きれい (綺麗 / 奇麗) "beautiful, clean"
- 嫌い (きらい) "disliked"
- 大嫌い (だいきらい) "strongly disliked"
- 幸い (さいわい) "fortunate"
- 得意 (とくい) "good at"
- 苦手 (にがて) "bad at"
Membership of this list varies a little across sources. The safe move is to treat it as a starter set rather than a closed inventory, and to fall back on the tests in the next subsection for any word in doubt.
One useful signal across most of the list is this: when the い is part of a kanji reading rather than a separable inflection slot, the word is almost always a な-形容詞. A regular い-形容詞 has its inflectional い written as kana outside the kanji (寒い, not 寒); a な-形容詞 ending in い has the い either inside the kanji reading (綺麗, 嫌い, 苦手) or as part of a fossilised verb-stem form.346
For きれい, the 大辞林 entry classifies the word as 形容動詞 (na-class). Orthographically, the second kanji's reading ends in い; the い is part of the 麗 reading, not a grammatical い hanging off a stem.34 嫌い is derived from the verb 嫌う ("to dislike"); its surface い is the fossilised stem-final vowel of the verb-derived form, which is one analysis among several but suffices to explain why 嫌い patterns as a な-形容詞 and nominalises so readily.46
きれいな花を買った。10
"I bought a beautiful flower."
納豆が嫌いです。10
"I dislike nattō."
苦手な科目は数学だ。11
"The subject I'm bad at is math."
The two reliable diagnostics
When the surface form is ambiguous, two tests decide the class. Either test is enough on its own.
Test 1: the attributive test. Place the word you are testing directly before a noun. If it links with no inserted morpheme, the word is an い-形容詞. If the link requires な, the word is a な-形容詞.110112
寒い日。10
"A cold day." (Attributive test on 寒い returns "i-adjective.")
静かな部屋。11
"A quiet room." (Attributive test on 静か returns "na-adjective.")
Test 2: the negative test. Form the negative. If the adjective inflects internally and the negative ends in 〜くない, the word is an い-形容詞. If the stem stays inert and the negative is built by following it with じゃない or ではない, the word is a な-形容詞.11011
大きくない。10
"It's not big." (Negative test on 大きい returns "i-adjective.")
きれいじゃない。10
"It's not beautiful." (Negative test on きれい returns "na-adjective.")
Both tests applied to きれい return "na-adjective": きれいな花, きれいじゃない. Applied to 嫌い, the same: 嫌いな食べ物, 嫌いじゃない. Applied to a regular い-形容詞 such as 大きい: 大きい本, 大きくない.1102
The point is to trust the tests, not the surface form.
What dictionaries and textbooks mark
Most learner resources flag adjective class as part of the entry metadata, so the tests above are rarely needed after the first few weeks of study.
- Jisho.org (drawing on the Electronic Dictionary Research and Development Group, or EDRDG, JMdict project) tags entries with
adj-i("I-adjective (keiyoushi)") andadj-na("Na-adjective (keiyodoshi)"). The いい / よい irregular subclass has its own tag,adj-ix.19 - Genki I labels each new adjective as "i-adj." or "na-adj." in the lesson-end vocabulary boxes, starting from Lesson 5.10
- Minna no Nihongo I marks adjective class with the superscript markers イ and ナ, introduced in Lesson 8 and used consistently for the rest of the series.11
- The JLPT N5 official problem set and the widely used "Tango N5" wordlist categorise each adjective entry by class as part of the standardised vocabulary infrastructure.1519
Once you know where the metadata lives, the tests become a fallback for the rare unmarked encounter, not a routine exercise.
Nuance and usage contexts
The two-class system has practical consequences that a conjugation table alone does not show: how each class predicates, how the な linker behaves, where the two classes tend to come from, and how some な-形容詞 double as nouns.
Adjectives as standalone predicates (no copula needed for い-形容詞)
An い-形容詞 in its dictionary form is already a complete predicate in plain register. 大きい is a grammatical sentence on its own and translates as "[it] is big." This is the central morphological difference between the two classes. い-形容詞 carry their own predicate marking and inflect for tense and polarity inside their own stem; な-形容詞 stems do not.11026
寒い。10
"It's cold."
The polite form 大きいです (or 寒いです) is the same predicate with a politeness marker stacked on the outside. The です has no syntactic function and does not make the predicate. This layered politeness use is endorsed in modern style guides, including NHK's broadcast handbook.1616
A な-形容詞 stem by itself is not a complete plain-register predicate. The speaker must supply the copula: 静かだ in plain register, 静かです in polite register.1212
One side effect of this asymmetry is that casual speech allows a bare-stem question such as 元気? ("doing OK?") with a な-形容詞. What is being dropped is the copula, not the adjective. There is no parallel bare-stem question for い-形容詞 because an い-形容詞 has no copula to drop.1718
The な link before a noun
The な of な-形容詞 is the attributive form of the copula. It attaches a copula-marked element to a following head noun. Historically, this な is a contraction of the classical attributive copula なる, preserved in modern Japanese specifically in this attributive slot.7176 The precise reconstruction of intermediate stages varies across sources. The safe summary is that な descends from a classical attributive copula form; the chain of intermediate shapes is a topic for a dedicated historical-grammar treatment.
The な is the same copula whose predicative form is だ. The two forms differ by syntactic slot, not by underlying word. A speaker who says 静かな部屋 at one point and この部屋は静かだ at another is using one copular pattern with two surface realisations.7126
This is why the same word that takes な before a noun takes だ at the end of a sentence. It is also why a な-形容詞 cannot modify a noun by simple juxtaposition; the copular linker has to be there.112
A handful of historically nari-class words survive as ren'taishi-like fossils. They take な only in attributive use and never appear predicatively in that shape. 大きな ("big") and 小さな ("small") are the two most common. They coexist with the regular い-形容詞 大きい and 小さい, and the な-marked forms are restricted to attributive use only.617
大きな夢。6
"A big dream."
The word 同じ ("same") is a recorded irregularity. It modifies a following noun directly without な (同じ人, "the same person"), but inserts な before some clause-linking particles (同じなので, 同じなのに). Historically 同じ inflected as a -shiku adjective and retains the adverbial form 同じく; the modern paradigm is mixed.36 The full treatment belongs to the sibling article on attributive-versus-predicative adjectives. Here, it is mentioned only as a recorded exception.
同じ人。3
"The same person."
同じなので、もう一度確認した。3
"Since it was the same, I checked once more."
Register and origin: a quick map
い-形容詞 tend toward native Yamato vocabulary covering sensory experience, everyday properties, and emotional states: 高い / 安い (price), 速い / 遅い (speed), 寒い / 暑い (temperature), 楽しい / 悲しい (emotion). The class is historically closed; the inventory available in modern Japanese is essentially the one inherited from Old Japanese with predictable phonological updates.596
な-形容詞 tend toward Sino-Japanese (漢語) vocabulary covering abstract qualities, institutional categories, and technical domains: 静か (calm), 有名 (famous), 自由 (free), 安全 (safe), 必要 (necessary), 重要 (important), 簡単 (simple). The class is open and absorbs new katakana loanwords as adjectival predicates (ハンサム, スマート, モダン, フレッシュ).967
This vocabulary tendency produces a register-frequency tendency. Conversational and light-fiction corpora over-represent い-形容詞 because they over-represent the native Yamato lexicon. Academic, legal, and technical corpora over-represent な-形容詞 because they over-represent the Sino-Japanese lexicon. The Balanced Corpus of Contemporary Written Japanese (BCCWJ) shows the imbalance directly across genres.20
The pattern is a tendency you can use to read register and predict gaps in your own vocabulary. It is not a classification rule. Native-origin な-形容詞 (静か, 豊か, 穏やか, 朗らか) and several emotional adjectives derived from verbs (好き, 嫌い) are na-class despite being Yamato in origin, so the two diagnostics from the identification section still decide individual cases.346
When な-形容詞 act like nouns (and when they do not)
A productive subset of な-形容詞 also function as nouns in their bare-stem form. This is especially common in the Sino-Japanese abstract-quality lexicon: 元気, 健康, 自由, 平和, 安全, 普通. The same word can appear as a na-adjective modifying a noun, or as the head of a noun phrase with a の modifier.9712
元気な人。10
"An energetic person."
元気の秘訣。7
"The secret of vitality."
To tell which job the word is doing in a given sentence, look at the linking particle on the modifier side. な points to the na-adjective reading (元気な人 = "the person is energetic"); の points to the noun reading (元気の秘訣 = "the secret of [the noun] vitality"). Both are grammatical for the words in this subset, and they are not synonyms.712
The dual-job behaviour is not universal across the na-class. Adjective-only な-形容詞 exist that do not have a productive noun reading (静か, 有名, 立派); for these words, の-marked attributive use is ungrammatical or strongly marked. The の-vs-な test identifies which job a given word is doing in a given sentence. It does not reclassify the whole class.712 A planned sibling article will handle the full debate. Here, it is treated only as a concept.
Good to know
The いい / 良い irregular is the one exception that matters
いい ("good") is the one common い-形容詞 whose inflected forms come from a different stem. Its dictionary form is いい, but every inflected form uses the older stem よ-: past よかった, negative よくない, te-form よくて, adverbial よく, past-negative よくなかった. The pattern is suppletion in the strict morphological sense, not a regular sound change.619
いい derives historically from the older adjective よい, itself from Old Japanese よし. Today, よい survives as a written and formal alternative dictionary form, while いい dominates spoken use. Only よい contributes the inflected forms.634
Jisho.org tags this class specifically as adj-ix to distinguish it from regular adj-i entries. Other adjectives that compound with いい (かっこいい, ちょうどいい) inherit the same suppletive pattern in their inflected forms (かっこよかった).196
The predictable production error is the regularised past form on the analogy of 大きい → 大きかった:
- Incorrect: いかった。
- Correct: よかった。
良かった。10
"(It) was good."
This is one of the few cases in N5 grammar where memorising the exception is the only fix.6 A dedicated sibling article will cover the full irregular paradigm. The takeaway here is to park the fact and not try to derive 良い's inflections from いい.
多い and the い-形容詞 that resist the attributive slot
多い ("many") is restricted in attributive use. Native speakers reject the bare attributive 多い人 or hear it as strongly marked.
The standard native equivalent for "many people" uses the connector の: 多くの人 (the adverbial form 多く plus の). The other natural workaround is to restructure the sentence so that 多い stands as the predicate, as in 人が多い ("there are many people").216
人が多い。10
"There are a lot of people."
多くの人が来た。6
"Many people came."
少ない ("few") shows a milder version of the same restriction, with 人が少ない as the unmarked predicative form.46 The restriction is a lexicalised quirk of a small set of quantity-denoting adjectives, not a general property of い-形容詞. The two tests from the identification section still correctly classify these words as い-class (negative: 多くない / 少なくない).61
Why some textbooks call them adjectival nouns
Descriptive linguistics labels な-形容詞 as "nominal adjectives," "adjectival nouns," or "copula-marked nouns." These labels capture the fact that their stems behave like nouns morphologically (no internal inflection, dependent on a copula) while their function is adjectival: ascribing a property to a referent.171312 School grammar (学校文法) labels the same class 形容動詞 ("adjective-verb"), reflecting the verb-like inflectional pattern that the copula gives the whole construction. The two labels point to the same word class from opposite ends of the same fact.32
The term 形容動詞 was coined by Haga Yaichi in the early twentieth century; before then the class had no unified school-grammar label.78
Both labels (形容動詞 and "nominal adjective" or "adjectival noun") appear in real learner materials and references. Neither is wrong; the underlying class is the same. The pedagogically dominant "na-adjective" label is a convenient surface-form description, not a competing analysis.172
Predicate adjectives carry tense, not the copula
For い-形容詞, tense and polarity are on the adjective stem, not on any following politeness marker. The past polite form is 寒かったです, not 寒いでした; the past negative polite is 寒くなかったです, not 寒くないでした. NHK's broadcast handbook explicitly endorses this layered usage in modern Japanese.1616
昨日は寒かったです。10
"It was cold yesterday."
For な-形容詞, tense and polarity are on the copula, not on the adjective stem. The past plain form is 静かだった, the past polite is 静かでした; the negative plain is 静かじゃない, the negative polite is 静かじゃないです or 静かではありません. The stem 静か does not change shape across the paradigm.11011
昨日は静かでした。11
"It was quiet yesterday."
The same English pattern, "it was cold" and "it was quiet," is delivered by morphologically opposite halves of the predicate. That is also why an i-adjective followed by です looks redundant only on the surface: です is doing politeness work and only politeness work. The past and negative have already been handled by the adjective stem.616
A note on writing: いい is usually kana, 良い is usually kanji
In modern written Japanese, the bare adjective "good" is overwhelmingly written いい in colloquial and spoken-register text. The kanji form 良い is reserved for more formal written register, and for compounds and set phrases (良い子 yoiko "good child," 良い知らせ "good news"). The inflected past 良かった is conventionally written with the kanji regardless of dictionary-register spelling.346
The split is by register, not by meaning. いい and 良い are the same word with different spelling conventions, and the spoken form has displaced the written form in most everyday text.36
For inflected forms, the kanji is conventional (良かった rather than いかった, よくない rather than いくない). The kana spelling of an inflected cell is non-standard for the same reason いかった is ungrammatical: the inflections derive from the よ- stem, and the kanji 良 is the conventional written form of that stem.46
See also
- Adjective Stem Nominalization in Japanese: ～さ vs. ～み
- Compound Adjectives in Japanese (複合形容詞): The V-Stem + Adjective Pattern
- Pitch Accent for Japanese Verbs and Adjectives: The Binary Class Rule and Conjugation Shifts
- How Japanese Grammar Works: A Big-Picture Overview