There are two types of adjectives in Japanese: i-adjectives and na-adjectives. The type of adjective is determined by its ending or—more precisely—the grammar that is required to join the adjective to nouns or transform the adjective into an adverb. Fortunately, it is obvious in almost all cases when looking at a new adjective whether it is an i-adjective or a na-adjective so you aren’t faced with the challenge of rote learning groups.

Here I’ll introduce i-adjectives and na-adjectives and their respective conjugations—yes, adjectives conjugate in Japanese! In English, adjectives themselves don’t transform when we talk in the negative, past, or past negative tense. In other words, in the following sentences the word “expensive” does not change.

The shoes are expensive.
The shoes are not expensive.
The shoes were expensive.
The shoes were not expensive.

The other thing you might notice is that some adjectives in Japanese are not adjectives in English. For instance, the most common way of saying “to like” uses “like” as an adjective.

I-Adjectives

All i-adjectives end in い which is written in hiragana. Below is a selection of JLPT N5 i-adjectives.

EnglishJapanese
Good
Cheapやす
Kindやさ
Lightあか
Newあたら
Busyいそが
Bigおお
Interesting面白おもしろ
Fun, enjoyableたの
Oldふる

The conjugation for i-adjectives always follows the same rules with just one exception: the adjective “good” (いい). Here you only have to remember that when the adjective conjugates into the past, negative, or past negative the first syllable becomes よ.

In fact, よい is the archaic word for good. It is still used today in formal writing but rarely in conversation in this standard form.

The other critical grammatical point to note is that we cannot add the standard form of the auxiliary verb to i-adjectives (this is the one exception to rule that all sentences must end with a verb in Japanese). For example, “this is cheap”:

これは やす。()
これは やすい。(✔)

Negative & Past Negative

For the negative and past negative we change the い to a く and then add the standard conjugation of the verb ある (the verb “to exist” for inanimate objects) which we met earlier.

EnglishDictionaryNegativePast Negative
to existあるないなかった

For example, to say “not expensive”:

たかい → たかく → たかくない

EnglishDictionaryNegativePast Negative
Cheapやすやすくないやすくなかった
Bigおおきいおおきくないおおきくなかった

Past Tense

For all other i-adjectives you simply change the final い to かった. Take the adjective for “tall” or “expensive” (たかい), for example:

たかい → たかかった

For the adjective “good” the past conjugation becomes:

いい → よかった

Polite Conjugations

For the polite conjugation we follow exactly the same rules as above, but add the polite form of the auxiliary verb: です.

StandardPolite
いいいいです
GoodGood
StandardPolite
よかったよかったです
Was goodWas good
StandardPolite
よくないよくないです
Is not goodIs not good
StandardPolite
よくなかったよくなかったです
Was not goodWas not good

For the negative and past negative we can also use the polite conjugation for ある instead.

EnglishPolite PresentPolite NegativePolite Past Negative
to existありますありませんありませんでした
Polite IPolite II
よくないですよくありません
Is not goodIs not good
Polite IPolite II
よくなかったですよくありませんでした
Was not goodWas not good

Both ways of conjugating to the negative and past negative are used in Japanese; however, the latter is arguably more polite.

Na-Adjectives

Na-adjectives can be conveniently defined as all those that don’t end with い with just a few exceptions.  The exceptions are “beautiful” (きれい), “hate” (きらい), and “grateful/happy” (さいわい) which look like い adjectives, but in fact conjugate as na-adjectives. Below is a selection of JLPT N5 na-adjectives.

EnglishJapanese
Like
Famous有名ゆうめい
Quietしず
Convenient便利べんり
Important, precious大切たいせつ
Skilful, good at上手じょうず
Unskilful, not good at下手へた
Necessary必要ひつよう
Splendid立派りっぱ
Seriousまじめ

Grammatically speaking, we must append the auxiliary verb to na-adjectives; however, in casual conversation this will very often be omitted.

わたしは 
I like (it).
わたしは きらい
I hate (it).

Because na-adjectives take the auxiliary verb we already know their conjugation. We just need to conjugate the auxiliary verb to get the negative, past, or past negative for both the standard and polite forms.

TenseStandardPolite
Presentきだです
Pastきだったでした
Negativeきではないではありません
Past Negativeきではなかったではありませんでした
わたしは きではありませんでした。
I didn’t like (it).

And, as we’ve seen before, in both cases we can replace では with the more colloquial じゃ.

わたしは じゃありませんでした。
I didn’t like (it).
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