Making an い-adjective polite is the easiest thing you will do in Japanese grammar: you take the plain form and add です. That is genuinely all there is to it — 高い（たかい） becomes 高いです, 面白い（おもしろい） becomes 面白いです. The catch, and the reason this page exists, is understanding what that です is doing. Here it adds politeness and nothing else. It is not a copula, it is not a verb, and it does not carry tense — misreading its job is the source of nearly every mistake English speakers make with adjectives.
The plain form is already a complete sentence
This is the single most important fact about い-adjectives, and English gives you no intuition for it. An い-adjective predicates on its own — it already contains the "is." You do not need a verb to finish the sentence.
このケーキ、おいしい。
kono kēki, oishii
This cake is delicious.
うわ、この部屋すごく暑い。
uwa, kono heya sugoku atsui
Whoa, this room is really hot.
Both of those are grammatically complete, natural sentences (informal). There is no missing word. Compare this with a noun: 学生（がくせい） "student" cannot stand alone as a sentence — 「彼は学生」 feels unfinished, and you must add だ or です to close it (彼は学生だ / 彼は学生です). The い-adjective needs no such help because the "is" is baked into the adjective itself.
Adding です for politeness
To speak politely, append です to the plain form. Because the adjective was already complete, です contributes register, not meaning.
この本はとても面白いです。
kono hon wa totemo omoshiroi desu
This book is very interesting.
今日は本当に暑いですね。
kyō wa hontō ni atsui desu ne
It's really hot today, isn't it.
駅から近いですか。
eki kara chikai desu ka
Is it close to the station?
Notice what does not happen: the い does not change, nothing is dropped, and です simply sits on the end. The proof that です is pure politeness is that you can delete it and still have a working sentence — おいしい and おいしいです mean the same thing; only the formality differs.
です is not a copula here — why 高いだ is wrong
Because です often translates as "is," learners reach for its plain-form partner だ and produce ×高いだ. This is one of the most common errors in all of beginner Japanese, and the logic above tells you exactly why it is wrong: the い-adjective already is the predicate, so there is nothing for a copula to do. だ only attaches to nouns and な-adjectives, which genuinely need a copula to predicate.
❌ この店のコーヒーは高いだ。
Incorrect — an い-adjective never takes だ; it already predicates.
✅ この店のコーヒーは高い。
kono mise no kōhī wa takai
This place's coffee is expensive. (plain)
✅ この店のコーヒーは高いです。
kono mise no kōhī wa takai desu
This place's coffee is expensive. (polite)
The asymmetry is worth memorizing as a pair: 高いだ is wrong, but 学生だ is right. The difference is not arbitrary — it falls straight out of the fact that adjectives self-predicate and nouns do not.
The polite negative: two options
The plain negative of an い-adjective is 〜くない (高い → 高くない). To make it polite you have two choices, and both are correct and common:
- 〜くないです — attach the politeness です to the plain negative. Slightly softer, very frequent in everyday polite speech.
- 〜くありません — a more formal-sounding form built on the verb ある. Common in careful or written-polite contexts.
この辺は駅からあまり近くないです。
kono hen wa eki kara amari chikakunai desu
This area isn't very close to the station.
いいえ、この店のラーメンは高くありません。
iie, kono mise no rāmen wa takaku arimasen
No, this restaurant's ramen isn't expensive. (formal)
| Register | "is not expensive" |
|---|---|
| plain | 高くない |
| polite (softer) | 高くないです |
| polite (more formal) | 高くありません |
The polite past: 〜かったです
The past tense of an い-adjective lives on the adjective, not on です. 寒い（さむい） → 寒かった → 寒かったです. The です stays in its plain, present shape; it never becomes でした after an い-adjective. This trips up English speakers constantly, because in English the tense sits on "was."
昨日は本当に寒かったです。
kinō wa hontō ni samukatta desu
Yesterday was really cold.
テストは思ったより難しくなかったです。
tesuto wa omotta yori muzukashikunakatta desu
The test wasn't as hard as I thought.
Since the adjective already carries the past (寒かった is "was cold" on its own), です once again only adds politeness. Writing ×寒いでした doubles up on the "is" and puts the tense in the wrong place.
| Meaning | Plain | Polite |
|---|---|---|
| is expensive | 高い | 高いです |
| is not expensive | 高くない | 高くないです / 高くありません |
| was expensive | 高かった | 高かったです |
| was not expensive | 高くなかった | 高くなかったです / 高くありませんでした |
Read that table as a single principle: tense and negation happen on the adjective; です only ever adds politeness. Once that clicks, all four polite forms are automatic.
How this differs from English
In English, "is / was" is a separate verb that carries both tense and (in a sense) the predication. Learners naturally look for a Japanese word to play that role and land on だ/です — hence ×高いだ and ×寒いでした. The mental correction is: an い-adjective is not "adjective + hidden be-verb," it is the verb. です is a courtesy word you clip on afterward, comparable to adding "sir" or a polite tone — it changes how you sound, not what you claim.
Common mistakes
❌ この本は面白いだ。
Incorrect — い-adjectives never take だ.
✅ この本は面白い。／ 面白いです。
kono hon wa omoshiroi / omoshiroi desu
This book is interesting. (plain / polite)
❌ 昨日は寒いでした。
Incorrect — tense goes on the adjective; です stays present-shaped.
✅ 昨日は寒かったです。
kinō wa samukatta desu
Yesterday was cold.
❌ テストは難しくなかったでした。
Incorrect — 〜かった already is the past; でした piles on a second past.
✅ テストは難しくなかったです。
tesuto wa muzukashikunakatta desu
The test wasn't hard.
❌ 部屋は明るいじゃないです。
Incorrect — じゃない is the noun/な-adjective negative; い-adjectives negate with くない.
✅ 部屋は明るくないです。
heya wa akarukunai desu
The room isn't bright.
The through-line in all four errors is the same misunderstanding: treating です as the thing that carries meaning and tense. It does not. It is the politeness cap, and the い-adjective underneath it does all the grammatical work. Get comfortable dropping です entirely in casual speech — 面白い, 寒かった, 高くない are all complete sentences — and the polite forms stop being a mystery, because you can see that です is just the extra layer on top.
Next, look at the one adjective that breaks the tidy pattern above: いい / よい, the irregular adjective.
Now practice Japanese
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Start learning Japanese→Related Topics
- i-Adjectives: PresentN5 — The dictionary form of an い-adjective ends in the kana い and works two ways with no helper word — straight before a noun (面白い本) and as a complete predicate ending a sentence (この本は面白い) — because the adjective already contains its own 'to be.'
- i-Adjectives: Negative (〜くない)N5 — To negate an い-adjective you drop the final い and add くない (高い→高くない); the polite versions are 〜くないです and 〜くありません — and crucially you never use じゃない, which belongs to nouns and な-adjectives.
- です: Polite PresentN5 — です as the polite non-past copula for nouns and na-adjectives — and, crucially, as a bare politeness marker on i-adjectives that already predicate, which is why the negatives differ (静かじゃないです vs 高くないです).