-이다

• (SK Standard/Seoul) IPA^(key): [ida̠]
• Phonetic hangul: [이다]

From Middle Korean 이〮라〮 (-í-lá), from Old Korean 是 (*i-). For the dialectal politeness particle use, compare 요 (-yo), Jeolla Korean 이라우 (-irau), both of which come from the copula.

이다 • (-ida) (infinitive 야 or 이야, sequential 이니)

1. to be; the Korean copula.

   Antonym: 아니다 (anida)

   사람은 동물이다. ― Saram-eun dongmur-ida. ― Humans are animals.

   이 나무는 큰 편이다. ― I namu-neun keun pyeon-ida. ― This tree is on the big side.

   전쟁은 바람직하지 않은 일이다. ― Jeonjaeng-eun baramjikha-ji an-eun ir-ida. ― War is undesirable business.

   악마도 한때 천사였다. ― Angma-do hanttae cheonsa-yeotda. ― The Devil was too once an angel.

• The declarative ending 다 (-da) changes to 라 (-ra) in some environments; see 라 (-ra) for details.
• In the informal polite form, 해요체 (haeyoche), after vowels 이다 (-ida) is often pronounced as 에요 (-eyo) instead of 예요 (-yeyo) in many environments.
• 이다 (-ida) is the Korean positive copula. Along with its negative, 아니다 (anida), it is sometimes categorized as an adjective, but more often placed in their own grammatical category distinct from both verbs and adjectives. Some grammars classify 이다 (-ida) as a particle and, like a particle, it is appended to the preceding word without an intervening space.
• For the existential meaning of "be", see 있다 (itda).

• The chart below is for consonant-final nouns.
• After a vowel-final noun, the initial vowel 이 (-i-) is deleted when followed by a syllable beginning with a single consonant (야 (-ya) included), except in quite formal language:

  돌이다 / 바다다 ― dor-i-da / bada-da ― it's a stone / it's the sea

  돌이야 / 바다야 ― dor-i-ya / bada-ya ― it's a stone / it's the sea

  적이겠다 / 친구겠다 ― jeog-i-get-da / chin'gu-get-da ― he's probably an enemy / he's probably a friend

• It is not normally deleted when followed by a consonant cluster, except in quite colloquial language:

  나무입니다 / 나뭅니다 ― namu-i-mnida / namu-mnida ― it is a tree / it is a tree (colloquial)

• It is not deleted when used as a noun or adnominal.

  나무인 / 나무임 ― namu-i-n / namu-i-m ― which is a tree / it being a tree

이다 • (-ida)

1. (eastern Jeolla dialect, southwestern Gyeongsang) A particle attached to intimate-, plain-, and semiformal-style verbs and adjectives to add a greater level of politeness.

   Synonyms: (Standard Seoul) 요 (-yo), (Jeolla) 이라우 (-irau)

• 다 (-da) — after vowels
• 이더 (-ideo), 더 (-deo) — after consonants

From Middle Korean 이다 (-i-ta).

이다 • (-ida)

1. Attached to certain ideophonic roots to derive verbs without frequentative effect.

   ‎펄럭 (peolleok, “fluttering”, ideophonic root) + ‎이다 (-ida) → ‎펄럭이다 (peolleogida, “to flutter”)

   ‎끄덕 (kkeudeok, “nodding”, ideophonic root) + ‎이다 (-ida) → ‎끄덕이다 (kkeudeogida, “to nod”)