True Adjectives: い-Adjectives (形容詞)
Master い-adjectives: self-contained native predicates that carry their own tense engines, dropping い to form past (〜かった), negative (〜くない), and adverbs (〜く), while mastering the irregular root いい (よい).
Describing with い-Adjectives (〜いです)
kono hon wa omoshiroi desu
This book is interesting.
Direct Modification vs Sentence Predicate
Micro-Shiftあれは 高い山です。
Modifying a noun directlyThat is a tall mountain.
Placed before a noun, the adjective directly describes its quality.
Example Sentences
Adjective Inflection Matrix
6 rules| Form | Ending change | Example | Note |
|---|---|---|---|
| Present Affirmative | 現在 (present)〜い / 〜いです | 高い(expensive)高い / いです | |
| Present Negative | 否定 (negative)〜くない / 〜くないです | 高い(not expensive)高くない | |
| Past Affirmative | 過去 (past)〜かった / 〜かったです | 高い(was expensive)高かった | |
| Past Negative | 過去否定 (past neg)〜くなかった / 〜くなかったです | 高い(was not expensive)高くなかった | |
| Adverbial Form | 副詞形 (adverb)〜く + Verb | 早い(run fast)早く走る | |
| The Irregular Pillar | いい (good)よくない · よかった · よくなかった | いい(good)よい (base) → よくない · よかった | *All inflections derive from the historical dictionary stem よい (yoi) |
Mental Model: The Autonomous Mini-Verb
Unlike English adjectives which require the verb “to be” (“The soup is cold”), Japanese い-adjectives are complete grammatical predicates on their own:
- In casual conversation, an い-adjective can end a sentence with zero helper words: 今日は 寒い。 = “Today is cold.” (Complete sentence!).
- In polite conversation, adding です does not function as a copula—it is merely a courtesy particle tacked on to show polite respect to your listener (寒いです = “It is cold (polite)”).
The Four Quadrants of Inflection
All inflection happens by replacing the final trailing hiragana い with dedicated suffixes:
- 1. Present Affirmative: Keep final い Casual: 高い (“expensive”) · Polite: 高いです.
- 2. Present Negative: Drop い, add 〜くない Casual: 高くない (“not expensive”) · Polite: 高くないです (or formal 高くありません).
- 3. Past Affirmative: Drop い, add 〜かった Casual: 高かった (“was expensive”) · Polite: 高かったです.
- 4. Past Negative: Drop い, add 〜くなかった Casual: 高くなかった (“was not expensive”) · Polite: 高くなかったです.
The Adverbial Engine: 〜く
To turn an い-adjective into an adverb that modifies a verb, drop い and attach 〜く:
- 早い (early/fast) → 早く 起きる (“wake up early”)
- 美味しい (delicious) → 美味しく 食べる (“eat with relish”)
- 安い (cheap) → 安く 買う (“buy cheaply”).
The Irregular King: いい (よい)
Japanese has only one irregular い-adjective: いい (“good”):
- Its dictionary root is historically よい.
- In the present affirmative, use いい or いいです.
- All other inflections must switch back to the yo-stem (よ-):
- Negative: よくない (never
*いくない!) - Past: よかった (never
*いかった!) - Past Negative: よくなかった
- Adverbial: よく (“well / frequently”).
In Dialogue
Kenji
Yuki
Tips and Common Traps
Quick Check
Choose the correct past affirmative form of the い-adjective おもしろい (interesting).