Complete Japanese Adjectives Conjugation Guide

The Two Classes of Japanese Adjectives

Unlike English, where adjectives have fixed shapes (e.g., "cold" is always "cold"), Japanese adjectives behave similarly to verbs—they conjugate to show tense, politeness, and negativity. Japanese adjectives fall into two distinct classes:

1. い-Adjectives (I-Adjectives / 形容詞, Keiyōshi): True inflecting adjectives that end natively in a hiragana **い**. They contain the copula function built-in and change their endings to reflect tenses.
   • Examples: 寒い (samui - cold), 高い (takai - high/expensive), 美しい (utsukushii - beautiful).
2. な-Adjectives (Na-Adjectives / 形容動詞, Keiyō-dōshi): Grammatical nouns that behave like adjectives. They require the connector particle **な (na)** when modifying nouns. They cannot conjugate on their own; instead, they rely on auxiliary verbs (like だ / です / でした) to express tense.
   • Examples: 静か (shizuka - quiet), 元気 (genki - healthy/energetic), きれい (kirei - clean/pretty). Note: きれい ends in 'i' but is a NA-adjective!

I-Adjective Conjugation Matrix

To conjugate an い-adjective, **drop the final い** and add the appropriate suffix. Adding **です** at the end turns casual forms into polite forms (without changing the grammatical suffix).

昨日、テストはとても難しかったです。
Kinō, tesuto wa totemo muzukashikatta desu.
"Yesterday, the test was very difficult." (Past polite conjugation of 難しい)

NA-Adjective Conjugation Matrix

Since な-adjectives function like nouns grammatically, they conjugate by changing the helper copula (だ / です) that follows them. The root word never changes.

昔、この町は静かでした。
"A long time ago, this town was quiet." (Past polite conjugation of 静か)

Adjective Noun Modification Rules

When placing an adjective directly in front of a noun to describe it, pay close attention to the grammar links:

• い-Adjectives connect **directly** to the noun. Never place の or だ between them.
  • 正しい用法: 美味しいりんご (oishii ringo - delicious apple)
  • 誤った用法: 美しいのだりんご ✗
• な-Adjectives must use the particle な (na) to link with the noun.
  • 正しい用法: 親切な人 (shinsetsu na hito - kind person)
  • 誤った用法: 親切だ人 ✗ / 親切人 ✗

Irregularities & The "ii" (Good) Exception

The Japanese word for "good" is いい (ii). However, this adjective originally derived from the classical form よい (yoi). This history created a unique grammatical exception:

• When using the non-past positive form, you can say いい or よい (いい is more common).
• For **every other conjugation**, you must use the base **よい (yoi)**. Conjugating from いい is grammatically incorrect.

Chaining Adjectives (Connecting Multiple Adjectives)

If you want to describe something with more than one adjective (e.g. "cheap and delicious"), you cannot just list them side by side. You must conjugate the preceding adjective into its **te-form**:

• For I-Adjectives: Drop the final い and add 〜くて (kute).
  • 安くて美味しい料理 (yasukute oishii ryōri - cheap and delicious food)
• For NA-Adjectives: Add 〜で (de).
  • 静かで綺麗な部屋 (shizuka de kirei na heya - quiet and clean room)

The final adjective in the chain determines the overall tense of the description: 安くて美味しかった (It was cheap and delicious).

Adverbial Upgrades & Nominalization

1. Turning Adjectives into Adverbs

You can turn adjectives into adverbs (e.g. "quick" → "quickly") to modify verbs:

• い-adjectives: Change the final い to く (ku).
  • 早い (hayai - quick) → 早く走る (hayaku hashiru - run quickly)
• な-adjectives: Add the particle に (ni) after the root.
  • 上手 (jōzu - skillful) → 上手にする (jōzu ni suru - do skillfully)

2. Turning Adjectives into Nouns (Nominalization)

To express measurements or degrees (e.g. "high" → "height", "deep" → "depth"), you can convert adjectives into abstract nouns:

• い-adjectives: Drop the final い and add 〜さ (sa) (objective measurement) or 〜み (mi) (subjective feeling).
  • 高い (high) → 高さ (height)
  • 深い (deep) → 深み (depth of flavor/feeling)

The "False I-Adjectives" Trap (NA-Adjectives Ending in 'I')

One of the most frequent grammar traps on the JLPT and in classroom tests is the group of な-adjectives that end in the sound 'i'. Because they end in 'i', many learners mistakenly conjugate them as い-adjectives (e.g. saying ✗ きれいかった instead of ✓ きれいだった). Here is a definitive list of these "false friends" and how to recognize them:

Universal Rule: If the adjective is written entirely in Kanji (like 有名) or is a Sino-Japanese word (consisting of two Kanji read with On'yomi readings), it is almost always a **な-adjective**, even if the final syllable sounds like 'i'. True い-adjectives usually have the **い** written outside the kanji as okurigana (e.g., 青い, 寒い, 詳しい).

Advanced Grammar Constructions with Adjectives

Japanese adjectives are highly active in building grammar patterns. Below are the four most common advanced structures you will encounter:

1. Showing Excessiveness: 〜すぎる (sugiru - Too Much)

To express that something is "too [adjective]," drop the final い (for I-adjectives) or the copula (for NA-adjectives) and attach the verb すぎる (which then conjugates as a Group 2 verb):

• い-adjective: 寒い (cold) → 寒 + すぎる = 寒すぎる (too cold)
• な-adjective: 静か (quiet) → 静か + すぎる = 静かすぎる (too quiet)
• Example: このお茶は熱すぎます。(This tea is too hot.)

2. Expressing Conjectures: 〜そう (sō - Looks Like / Seems)

To say that something "looks like" or "seems to be" a certain way based on visual evidence, drop the final い or the copula and add そう:

• い-adjective: 美味しい (delicious) → 美味し + そう = 美味しそう (looks delicious)
• な-adjective: 元気 (healthy) → 元気 + そう = 元気そう (looks healthy)
• Special Exceptions:
  • いい (good) becomes よさそう (looks good).
  • ない (non-existent/not) becomes なさそう (seems not to be).
• Example: 彼は忙しそうです。(He looks busy.)

3. Describing Third-Person Emotions: 〜がる (garu - To Show Signs Of)

In Japanese, you cannot directly state another person's subjective feelings (like "he is cold" or "she wants a car") because you cannot read their mind. Instead, you must describe their outward signs by dropping the final い and adding がる (conjugates as a Group 1 verb):

• い-adjective: 寒い (cold) → 寒 + がる = 寒がる (to behave as if cold / complain about cold)
• Desire particle: 欲しい (want) → 欲し + がる = 欲しがる (to show signs of wanting)
• Example: 犬が外に出たがっています。(The dog wants to go outside — showing signs of wanting to go out.)

4. Initiating Actions/Changes: 〜にする (ni suru - To Make Something [Adjective])

To express making an object change its state, use the adverbial form of the adjective combined with the verb **する**:

• い-adjective: 大きい (big) → 大きく + する = 大きくする (to make bigger)
• な-adjective: 綺麗 (clean) → 綺麗に + する = 綺麗にする (to clean / make clean)
• Example: テレビの音を小さくしてください。(Please turn down/make smaller the TV volume.)

Politeness Registers for Negatives

In polite speech, there are two ways to conjugate the negative forms of both adjective classes. One is standard polite, and the other is slightly stiffer, more formal, and preferred in business or writing:

For past negative polite expressions, the formal forms follow the same logic: 高くありませんでした (formal past negative) vs. 高くなかったです (standard polite past negative).

Frequently Asked Questions

Related Study Guides

• Japanese Verb Conjugation Guide
• Japanese Particles Master Guide
• Complete JLPT N5 Grammar Guide
• Japanese Conditionals Master Guide