아니다 (to not be): Negative Copula Paradigm

To say what something is, Korean uses the copula 이다; to say what it is not, it does not glue a general "not" onto 이다 — it reaches for a completely separate word, 아니다. This is a suppletive pair, exactly like English good → better rather than good → gooder: there is no ×안 이다. This page is the full look-up table for 아니다 across the four speech levels, plus the one structural fact that matters more than any single form — the noun you are denying takes the subject particle 이/가, never 을/를.

What kind of word 아니다 is

아니다 has the stem 아니- and conjugates as a descriptive verb (an "adjective" in the Korean sense — a verb that describes a state rather than an action). That single fact predicts almost everything below:

• It takes no imperative and no propositive — you cannot command or propose "not-being" something (there is no ×아니세요, ×아니자). Descriptive verbs never do.
• Its present attributive is 아닌 (the -(으)ㄴ adjective modifier), not ×아니는: 학생이 아닌 사람 ("a person who is not a student").
• Its complement takes 이/가, because descriptive verbs have no object to mark with 을/를.

One spelling landmine lives in the stem itself: the past is 아니었어요, written with 었 — not ×아니였어요. The 였 spelling belongs to the vowel-final copula (친구였어요), not to 아니다.

The full paradigm

Rows are tense/mood; columns are the four speech levels. Each cell shows the Hangul with its Revised-Romanization reading beneath.

Note that 반말 and 한다체 present differ: casual speech says 아니야 (or the contraction 아냐), while the plain/written declarative is simply the dictionary form 아니다 — the shape you meet in books, headlines, and diaries. That plain 아니다 is why the "dictionary form" and the "written statement" look identical for descriptive verbs.

The 이/가 complement: the whole game

Every 아니다 sentence has the same skeleton:

A 은/는   B 이/가   아니다

A is the topic (은/는); B — the thing being denied — wears the subject particle 이/가, chosen by B's final sound: 이 after a consonant, 가 after a vowel.

학생, 사람, 잘못 end in consonants → 이 (학생이, 사람이, 잘못이); 의사, 남자친구 end in vowels → 가 (의사가, 남자친구가). The reframing English speakers need is subtle: in "X is not Y," English feeds Y straight into the verb as a kind of object, but Korean treats Y as if it were a second subject — "as-for-X, Y isn't-the-case." That is why the negated noun is marked exactly like a subject.

The forms in real sentences

The last two show 아니다's two "hard" corners: the attributive 아닌 turning a whole clause into a noun-modifier (한국 사람이 아닌 친구), and the bare 한다체 아니다 closing a written statement. Both are everyday once you notice that 아니다 simply behaves like any other descriptive verb (좋다 → 좋은, 좋다).

Common Mistakes

1. Marking the complement with 을/를. 아니다 is a descriptive verb; the denied noun takes 이/가.

2. Building "is not" as 안 + 이다. The copula has a suppletive negative; there is no ×안 이에요.

3. Misspelling the past as 아니였어요. The stem is 아니-, and the past is 아니었어요, with 었.

4. Picking the wrong 이/가 allomorph. 이 after a consonant, 가 after a vowel.

Key Takeaways

• 아니다 is the suppletive negative of 이다 — a single word meaning "is not [X]," not 안 + 이다.
• It conjugates as a descriptive verb (stem 아니-): no imperative/propositive, present attributive 아닌, plain declarative 아니다.
• The frame is A은/는 B이/가 아니다 — the denied noun B takes 이/가 (이 after a consonant, 가 after a vowel), never 을/를.
• Watch the spelling seams: past 아니었어요 (not ×아니였어요), and — on the 이에요/예요 page — present 아니에요 (not ×아니예요), because 아니 has no noun-final 이 to fuse into 예.