です is the single most-used word in polite Japanese, and almost every learner meets it in their first lesson as "is / am / are." That gloss is good enough to get started, but it hides a structural fault line that, once you see it, dissolves a surprising amount of beginner confusion. です actually does two different jobs depending on what sits in front of it: after a noun or na-adjective it is the copula, but after an i-adjective it is only a politeness dressing on a word that already stands as a complete predicate. This page pulls those two jobs apart.
です as the copula: nouns
After a noun, です is the real thing — the polite non-past copula. It asserts "[topic] is [noun]" and raises the sentence to polite register.
私は学生です。
watashi wa gakusei desu
I am a student.
田中さんは医者です。
tanaka-san wa isha desu
Tanaka is a doctor.
こちらは私の妻です。
kochira wa watashi no tsuma desu
This is my wife.
Here です is doing the predicating: strip it away and you are left with a bare noun (学生, 医者) that cannot stand as a sentence. This is the copula in its purest form.
です as the copula: na-adjectives
na-adjectives (静か, 便利, 元気, きれい, 好き) behave like nouns, so they too take です as their genuine copula. Note there is no な before です — the な only appears when a na-adjective sits directly in front of a noun (静かな部屋), never before the copula.
この部屋は静かです。
kono heya wa shizuka desu
This room is quiet.
この駅はとても便利です。
kono eki wa totemo benri desu
This station is very convenient.
日本の料理が好きです。
nihon no ryōri ga suki desu
I like Japanese food.
です as pure politeness: i-adjectives
Now the crucial difference. i-adjectives (高い, 面白い, 忙しい, おいしい) are self-predicating: 高い all by itself is already a complete plain sentence, "[it]'s expensive." So when you attach です to an i-adjective, です is not functioning as a copula — the word in front of it needs no help predicating. です here is a pure politeness marker, adding register and nothing else.
この本は面白いです。
kono hon wa omoshiroi desu
This book is interesting.
今日は暑いですね。
kyō wa atsui desu ne
It's hot today, isn't it.
あのラーメン、おいしいですよ。
ano rāmen, oishii desu yo
That ramen is delicious, you know.
The single most useful consequence: because the i-adjective already predicates, its plain form takes no copula at all. The plain version of 面白いです is simply 面白い — not ×面白いだ. Learners who expect a copula everywhere invent ×面白いだ by analogy with 学生だ, but there is no slot for it. (See だ: plain form for the flip side of this rule.)
Why the two jobs matter: negatives split
The clearest proof that です does two different things is what happens when you make each type negative. Because a noun/na-adjective relies on the copula, its negative reshapes the copula. Because an i-adjective predicates on its own, its negative reshapes the adjective, and です just rides along on top.
| Type | Affirmative | Negative (casual polite) | Negative (more formal) |
|---|---|---|---|
| Noun | 学生です | 学生じゃないです | 学生ではありません |
| na-adjective | 静かです | 静かじゃないです | 静かではありません |
| i-adjective | 高いです | 高くないです | 高くありません |
Look at the pattern. Nouns and na-adjectives negate the copula itself (じゃない / ではありません). The i-adjective instead changes its own ending (高い → 高くない) and then, optionally, adds です back on for politeness. There is no ×高いじゃない, because 高い is not a noun that a copula could negate.
彼は学生じゃないです。
kare wa gakusei ja nai desu
He isn't a student.
この問題は難しくないです。
kono mondai wa muzukashiku nai desu
This problem isn't difficult.
ここはあまり静かではありません。
koko wa amari shizuka de wa arimasen
It's not very quiet here.
The negative copula has its own dedicated page — see negative: じゃない / ではない.
Orthography note: keep the い before です
An i-adjective keeps its full kana ending — the okurigana い — right before です. The い is part of the word, not something the copula swallows.
この店は安いです。
kono mise wa yasui desu
This shop is cheap.
Write 安いです, never ×安です. Likewise 面白いです, not ×面白です. The i-adjective's dictionary shape is untouched; です simply follows it. For the full treatment of polite i-adjectives, see i-adjectives with です.
Common Mistakes
1. Attaching だ to an i-adjective. i-adjectives are self-predicating and take no copula. The plain form is the bare adjective.
❌ このかばんは高いだ。
kono kaban wa takai da
Wrong — no だ on an i-adjective.
✅ このかばんは高い。／ 高いです。
kono kaban wa takai / takai desu
This bag is expensive. (plain / polite)
2. Negating an i-adjective with じゃない. じゃない negates a noun or na-adjective copula, not an i-adjective. The i-adjective changes its own ending to くない.
❌ この本は面白いじゃないです。
kono hon wa omoshiroi ja nai desu
Wrong — you can't negate an i-adjective with じゃない.
✅ この本は面白くないです。
kono hon wa omoshiroku nai desu
This book isn't interesting.
3. Botching the okurigana in the i-adjective negative. The stem drops the final い before くない — it is 面白 + くない, not 面白い + くない.
❌ この本は面白いくないです。
kono hon wa omoshiroiku nai desu
Wrong okurigana — the い is replaced, not kept.
✅ この本は面白くないです。
kono hon wa omoshiroku nai desu
This book isn't interesting.
4. Dropping the い before です. The i-adjective keeps its full form; です is added on top, not fused in.
❌ この店は安です。
kono mise wa yasu desu
Wrong — the い of 安い is part of the word.
✅ この店は安いです。
kono mise wa yasui desu
This shop is cheap.
5. Inserting な before です with a na-adjective. The な appears only before a noun (静かな部屋), never before the copula.
❌ この部屋は静かなです。
kono heya wa shizuka na desu
Wrong — no な before です.
✅ この部屋は静かです。
kono heya wa shizuka desu
This room is quiet.
Key Takeaways
- After a noun or na-adjective, です is the polite copula (学生です, 静かです).
- After an i-adjective, です is politeness only (面白いです) — the adjective already predicates, so its plain form is the bare 面白い, never ×面白いだ.
- The negatives split accordingly: noun/na → 〜じゃないです／ではありません; i-adjective → 〜くないです／くありません. Never ×高いじゃない.
- Orthography: keep the い (安いです, not ×安です); the negative replaces it (安くない, not ×安いくない).
- No な before です; the な only links a na-adjective to a following noun.
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Start learning Japanese→Related Topics
- だ: Plain Form and When to Drop ItN5 — The plain-form copula だ and the two-layer rule for when it appears — a grammar layer (obligatory before と, から, けど; forbidden before か and question の) and a register layer (freely dropped in casual noun predicates).
- Negative: じゃない / ではないN5 — The negative copula on a register ladder — casual じゃない, written ではない, polite じゃありません / ではありません — plus why na-adjectives negate the same way but i-adjectives never do (高くない, never ×高いじゃない).
- i-Adjectives with ですN5 — How to make い-adjectives polite by adding です, including the polite negative and polite past.