I-Adjectives in Japanese

In Brief

Japanese い-adjectives usually end in い and can describe a noun directly: 大きい犬 means “a big dog.” They also change their own ending for negation and past time: 高い “is expensive,” 高くない “is not expensive,” and 高かった “was expensive.” In polite speech, add です after the correctly conjugated adjective: 高くないです, not 高いではありません.

Overview

An adjective is a word that describes a noun (a person, place, thing, or idea). Japanese has two main adjective classes, and this article covers い形容詞 (i-keiyōshi), or い-adjectives. Common examples include 大きい “big,” 新しい “new,” 暑い “hot,” and 面白い “interesting.” This is an A1 foundation: it lets you talk about prices, weather, food, places, people, and experiences.

The main surprise for an English-speaking learner is that an English adjective stays unchanged while a separate verb carries tense: “is cold,” “was cold,” “is not cold.” A Japanese い-adjective changes its own ending instead: 寒い, 寒かった, 寒くない. For that reason, it is useful to learn each adjective as a word that can conjugate, not merely as a label placed next to a noun.

In a polite sentence, です follows the adjective, as in この本は面白いです. However, です does not replace the adjective’s conjugation. To say “was interesting,” change 面白い to 面白かった first, then add です: 面白かったです.

How It Works

Recognizing the class

The dictionary form of a regular い-adjective ends in the hiragana い. Even when the word is normally written with kanji, that final い remains visible and is the part that changes.

Dictionary form	Reading	Meaning
大きい	おおきい (ōkii)	big
小さい	ちいさい (chiisai)	small
高い	たかい (takai)	high; tall; expensive
安い	やすい (yasui)	inexpensive
新しい	あたらしい (atarashii)	new
面白い	おもしろい (omoshiroi)	interesting; funny
暑い	あつい (atsui)	hot weather
長い	ながい (nagai)	long

The ending alone is not a perfect test. Some な-adjectives happen to end in the sound い, notably きれい “pretty; clean” and 嫌い “disliked.” They follow a different pattern: きれいな部屋, not きれいい部屋 or きれい部屋. Check a dictionary when unsure.

Before a noun

Put an い-adjective directly before a noun (a naming word such as “dog” or “book”):

い-adjective + noun

• 大きい犬 — a big dog
• 新しい本 — a new book
• 安いホテル — an inexpensive hotel

Do not insert の, な, or です between the adjective and noun. Japanese normally has no equivalent of English “a” or “the” here; context determines whether 大きい犬 means “a big dog” or “the big dog.”

A conjugated negative or past form can also modify a noun:

• 高くない店 — a shop that is not expensive
• 面白かった映画 — a movie that was interesting

At the end of a sentence

An い-adjective can be the predicate (the part that says what the topic is like):

• この本は面白い。 — This book is interesting. (plain)
• この本は面白いです。 — This book is interesting. (polite)

The topic is often marked with は. Japanese does not require a separate word meaning “is” in the plain sentence. Add です for polite style, but do not add plain だ after an い-adjective: 面白いだ is incorrect.

The four core forms

For a regular adjective such as 高い, remove the final い and attach the required ending.

Meaning	Ending	Plain form	Polite form
present affirmative	い	高い	高いです
present negative	くない	高くない	高くないです
past affirmative	かった	高かった	高かったです
past negative	くなかった	高くなかった	高くなかったです

Here “present” means non-past: it can describe something true now or generally true. Context supplies the time.

The building process is regular:

• 高い → 高くない: not high / not expensive
• 高い → 高かった: was high / was expensive
• 高い → 高くなかった: was not high / was not expensive

The same pattern applies to 暑い → 暑くない → 暑かった → 暑くなかった and 新しい → 新しくない → 新しかった → 新しくなかった.

Polite negative alternatives

Both 高くないです and 高くありません mean “is not expensive” and are polite. 〜くないです is very common in conversation. 〜くありません sounds somewhat more formal. Their past-negative forms are:

• 高くなかったです — was not expensive
• 高くありませんでした — was not expensive (more formal)

Notice that でした is possible in ありませんでした, but not directly after the dictionary-form adjective. 高いでした is wrong.

The important irregular adjective いい

いい “good” is irregular in every form that replaces its final い. Its conjugated forms use the older stem よ-.

Meaning	Form
is good	いい / よい
is not good	よくない
was good	よかった
was not good	よくなかった

Therefore, say よかったです “it was good,” not いかったです. Compounds ending in いい generally behave the same way: かっこいい “cool; good-looking” becomes かっこよくない and かっこよかった.

Asking questions

In polite speech, add か to the polite sentence:

• この本は面白いですか。 — Is this book interesting?
• 昨日は暑かったですか。 — Was it hot yesterday?

In casual conversation, rising intonation can turn a plain form into a question: 高い？ “Is it expensive?” Start with ですか until you are comfortable choosing an appropriate level of politeness.

Examples in Context

Japanese	English	Note
この本は面白いです。	This book is interesting.	Polite present predicate
今日は暑くないです。	It is not hot today.	Present negative
映画は長かったです。	The movie was long.	Past affirmative
昨日のテストは難しくなかったです。	Yesterday’s test was not difficult.	Past negative
大きい犬が公園にいます。	There is a big dog in the park.	Directly before a noun
新しい靴を買いました。	I bought new shoes.	The adjective modifies 靴
このホテルは安いですか。	Is this hotel inexpensive?	Polite question
あの山は高いです。	That mountain is high.	高い describes height
このかばんは高くありません。	This bag is not expensive.	More formal polite negative
天気はよかったです。	The weather was good.	Irregular いい → よかった
その店はよくなかったです。	That shop was not good.	Irregular past negative
面白かった映画をもう一度見ました。	I watched the interesting movie again.	Past form before a noun
部屋は広くて、明るいです。	The room is spacious and bright.	くて links descriptions

The English translation may need “is,” “was,” “a,” or “the,” but those words do not each correspond to a separate element in the Japanese sentence.

Common Mistakes

Leaving the adjective unchanged in the negative

• Wrong: 今日は暑いないです。
• Right: 今日は暑くないです。
• Why: Remove final い and add くない. Do not simply put ない after the complete dictionary form.

Putting でした after the dictionary form

• Wrong: 映画は長いでした。
• Right: 映画は長かったです。
• Why: Past time is marked on the い-adjective with かった. です then adds politeness.

Adding な or の before a noun

• Wrong: 大きいな犬 / 大きいの犬
• Right: 大きい犬
• Why: An い-adjective attaches directly to the noun. な belongs to the attributive pattern of な-adjectives, while の commonly links nouns.

Treating every word ending in い as an い-adjective

• Wrong: きれくない部屋
• Right: きれいではない部屋 / きれいじゃない部屋
• Why: きれい is a な-adjective despite its final sound. Its negative is not formed by replacing the last い with くない.

Conjugating いい regularly

• Wrong: いくない / いかった
• Right: よくない / よかった
• Why: Conjugated forms of いい use the stem よ-.

Adding だ after an い-adjective

• Wrong: この本は面白いだ。
• Right: この本は面白い。 / この本は面白いです。
• Why: A plain affirmative い-adjective ends the sentence by itself. Use です, not だ, when politeness is needed.

Usage Notes

Plain and polite speech

Forms without です are normal in casual conversation, diary-style writing, and speech to close friends: 寒い, 寒くない, 寒かった. Forms with です are safe in most everyday polite interactions. Keep one style consistent within a simple sentence rather than switching levels without a reason.

Meanings depend on what is described

Some common adjectives cover several English words. 高い can mean “high,” “tall,” or “expensive”; the noun and situation choose the meaning. 面白い can mean “interesting” and sometimes “funny.” 熱い describes something hot to the touch, such as tea, while 暑い describes hot weather or an uncomfortably hot environment. Learning an adjective with a typical noun prevents many vocabulary errors.

Subjects are often omitted

Japanese commonly leaves out a topic or subject when listeners can recover it from context. A speaker looking at a price tag may simply say 高いですね: “It’s expensive, isn’t it?” Do not force an English pronoun such as “it” into every Japanese sentence.

Beyond the Basics / Advanced Use

You do not need to master the following patterns at A1, but they explain forms you will soon encounter.

Linking descriptions with 〜くて

To connect an い-adjective to another description, replace final い with くて:

• この町は小さくて、静かです。 — This town is small and quiet.
• この料理は安くて、おいしいです。 — This dish is inexpensive and delicious.

The connection often resembles “and,” but context can also suggest a reason: 暑くて、眠れませんでした means “It was hot, so I couldn’t sleep.” Do not use そして mechanically between every pair of adjectives.

Adverbial 〜く

Changing final い to く can make an adverbial form, a form that describes how an action happens:

• 早い → 早く起きます。 — I get up early.
• 大きい → 大きく書いてください。 — Please write it large.

This is developed further under adjectives used as adverbs.

Turning a quality into a noun with 〜さ

Replacing final い with さ creates a noun referring to the degree or quality itself:

• 高さ — height
• 長さ — length
• 暑さ — heat

For example, 山の高さ means “the height of the mountain.” This is productive, but the resulting English translation is not always a word ending in “-ness.”

〜そう for appearance

A later pattern uses the adjective stem plus そう to mean that something looks a certain way: おいしそう “looks delicious.” There are special details, including いい → よさそう and the distinction between “does not look...” and “looks not...,” so learn this as its own grammar point rather than treating そう as a simple universal ending.

Fixed forms and apparent exceptions

大きな and 小さな occur before nouns, as in 大きな問題 “a big problem.” They resemble な-adjectives but are special attributive forms: they cannot normally end a sentence as 大きなです. The ordinary い-adjectives 大きい and 小さい remain fully regular.

You may also see classical or literary forms, and set expressions that do not match the basic modern paradigm. These do not change the beginner rule: for ordinary modern い-adjectives, learn い → くない / かった / くなかった.

Practice Tips

1. Drill four forms as a set. Choose five useful adjectives and say all four forms aloud: 新しい、新しくない、新しかった、新しくなかった. Include いい every time so its irregular stem becomes automatic.
2. Describe one object in two positions. Pair a noun phrase with a sentence: 安い店 “an inexpensive shop” and この店は安いです “this shop is inexpensive.” This reinforces that no な or の is needed.
3. Use a daily time contrast. Write one sentence about today and one about yesterday: 今日は寒くないです。昨日は寒かったです。 The time word gives you a clear reason to choose the non-past or past form.

Related Concepts

• Prerequisite: Copula です/だ — helps you understand politeness and why い-adjectives do not behave like nouns.
• Next step: Adjectives as Adverbs — develops the 〜く form used to describe actions.
• Next step: Degree Adverbs — adds expressions such as “very,” “a little,” and “not very.”
• Next step: Basic Comparison より — compares people and things using adjectives.
• Later topic: Appearance そう — expresses that something looks hot, tasty, easy, and so on.