形容詞の変化

Adjective Conjugation
There are two classes of Japanese adjectives referred to in this document as い-adjectives and な-adjectives. い-adjectives are actually a special class of verb and therefore require no predicate.  They do however require conjugation.  な-adjectives, also called copular nouns, are actually nouns and need no conjugation, but do require a predicate in order to activate their adjectival function.

The Conjugations

い-adjectives
In the following chart showing the conjugation of the い-adjective 寒い [samui, cold] notice that when い-adjectives are informal they require no predicate. You will never see an い-adjective followed by だ. However to make い-adjectives polite you would add です.  

Informal (Common 普段)
寒い Present Affirmative is/are cold
寒くない Present Negative is not/are not cold
寒かった Past Affirmative was/were cold
寒くなかった Past Negative was not/were not cold
寒かろう Tentative is/are probably cold
 
Formal (Polite 丁寧)
寒いです Present Affirmative is/are cold
寒くないです Present Negative is not/are not cold
寒かったです Past Affirmative was/were cold
寒くなかったです Past Negative was not/were not cold
寒いでしょう Tentative is/are probably cold
 
Conditional and Conjunctive
寒ければ Conditional if [subject] is/was cold
寒くなければ Negative Conditional if [subject] is not/was not cold
寒くて Conjunctive is/are cold ... and/so
寒くなくて Negative Conjunctive is not/are not cold ... and/so
 
Keep in mind though that です would only be used when the い-adjective is at the end of a sentence and only to make it polite.  This is because い-adjectives are a special type of verb so even without だ/です the meaning of 寒い is "is cold" and not just "cold."  If an い-adjective is used to modify something in the middle of a sentence, you would use no predicate regardless of the level of politeness.  Here are some examples to clarify this point:

Informal Polite
寒い天気だ。
[samui tenki da.]
It's cold weather. (adjective in the middle)
寒い天気です。
[samui tenki desu.]
It's cold weather. (adjective in the middle)
お天気は寒い。
[otenki wa samui.]
The weather is cold. (adjective at the end)
お天気は寒いです。
[otenki wa samui desu.]
The weather is cold. (adjective at the end)

There is one irregular case, which is いい/よい [ii/yoi, good].  With this い-adjective you would use いい or よい for present affirmative but use only よい for conjugating all other tenses like so:

Informal (Common 普段)
いい Present Affirmative is/are good
よくない Present Negative is not/are not good
よかった Past Affirmative was/were good
よくなかった Past Negative was not/were not good
よかろう Tentative is/are probably good
 
Formal (Polite 丁寧)
いいです Present Affirmative is/are good
よくないです Present Negative is not/are not good
よかったです Past Affirmative was/were good
よくなかったです Past Negative was not/were not good
よいでしょう Tentative is/are probably good
 
Conditional and Conjunctive
よければ Conditional if [subject] is/was good
よくなければ Negative Conditional if [subject] is not/was not good
よくて Conjunctive is/are good ... and/so
よくなくて Negative Conjunctive is not/are not good ... and/so


な-adjectives
In this next chart showing the conjugation of the な-adjective 綺麗な [kireina, pretty] notice that unlike い-adjectives, when な-adjectives are informal and at the end of the sentence they are followed by だ.  な-adjectives must always be followed by a predicate or by -な.  

Informal (Common 普段)
綺麗だ Present Affirmative is/are pretty
綺麗ではない Present Negative is not/are not pretty
綺麗だった Past Affirmative was/were pretty
綺麗ではなかった Past Negative was not/were not pretty
綺麗だろう Tentative is/are probably pretty
 
Formal (Polite 丁寧)
綺麗です Present Affirmative is/are pretty
綺麗ではありません Present Negative is not/are not pretty
綺麗でした Past Affirmative was/were pretty
綺麗ではありませんでした Past Negative was not/were not pretty
綺麗でしょう Tentative is/are probably pretty
 
Conditional and Conjunctive
綺麗だったら Conditional if [subject] is/was pretty
綺麗ではなかったら Negative Conditional if [subject] is not/was not pretty
綺麗で Conjunctive is/are pretty ... and/so
綺麗ではなくて Negative Conjunctive is not/are not pretty ... and/so

Like い-adjective だ/です would only be used when a な-adjective is at the end of a sentence but this time they are required because な-adjectives are a type of noun so だ/です is used to make the meaning of 綺麗 into "is pretty" and not just "pretty."  If a な-adjective is used to modify something in the middle of a sentence, you would use the な-adjective root followed by -な. Here are some examples:

Informal Polite
佐藤さんは綺麗な人だ。
[satousan wa kireina hito da.]
Mrs. Sato is a pretty person. (adj. in the middle)
佐藤さんは綺麗な人です。
[satousan wa kireina hito desu.]
Mrs. Sato is a pretty person. (adj. in the middle)
佐藤さんは綺麗だ。
[satousan wa kirei da.]
Mrs. Sato is pretty. (adjective at the end)
佐藤さんは綺麗です。。
[satousan wa kirei desu.]
Mrs. Sato is pretty. (adjective at the end)

Some Practice Adjectives

Here is a list of adjectives that you can use to practice the conjugations we have gone over thus far.  Pay particular attention to the differences in the usage of だ/です with い-adjectives and な-adjectives.  Also note how both types of adjectives differ when used in the middle of a sentence or at the end of the sentence.  It might be beneficial to learn the conjugations by drilling them first, then practice using them in sentences, writing them down on paper and saying them out loud.

い-adjectives
暖かい あたたかい warm
熱い あつい hot
痛い いたい painful
美しい うつくしい beautiful
固い かたい hard (vs. soft)
厳しい きびしい harsh
素晴らしい すばらしい wonderful
鋭い するどい sharp
高い たかい tall, high
正しい ただしい right, proper
長い ながい long
低い ひくい short (stature)
短い みじかい short (length)
難しい むずかしい difficult
優しい やさしい easy
安い やすい cheap, inexpensive
柔らかい やわらかい soft

な-adjectives
鮮やかな あざやかな fresh
哀れな あわれな pitiful, wretched
異常な いじょうな abnormal
意地悪な いじわるな mean
嫌な いやな distasteful
頑固な がんこな stubborn
簡単な かんたんな easy
静かな しずかな quiet
好きな すきな likeable, favorite
丁寧な ていねいな polite
生意気な なまいきな conceited, impudent
派手な はでな showy, cheeky
不思議な ふしぎな mysterious
不便な ふべんな inconvenient
変な へんな strange
便利な べんりな convenient
真面目な まじめな serious
野蛮な やばんな savage, barbarous

After each conjugation you write stop and think about how you would use it in a sentence.  Try to relate each conjugation to the essence of their English equivalent.  After some time, the nuance of time, negativity and the level of politeness conveyed by these adjective endings will become second nature to you and you will be able to drop the use of English equivalents (of course finding English equivalents for levels of politeness is a little difficult since 熱い and 熱いです both mean it's hot in English.  The level of politeness to use can only be felt out through observing what levels of politeness Japanese people themselves use in different situations.  However a good rule of thumb is to use informal with family and friends, and polite with strangers and in formal situations).

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