Morphological Causative -이-

Korean's oldest and tightest way to say "cause something to happen" is not a separate verb but a suffix wedged inside the verb — one of seven causative infixes 이/히/리/기/우/구/추 that sit between the stem and the ending. This page covers the first and most common of them, -이-. Insert it into 먹다 ("eat") and you get 먹이다 ("feed"); into 죽다 ("die") and you get 죽이다 ("kill"). The mechanics are simple. The genuinely hard part — which this page is honest about — is that which of the seven suffixes a given verb takes is fixed by tradition, not derivable by rule, so -이- has to be learned verb by verb.

How -이- builds a causative

Take the dictionary stem, drop the 다, insert 이, and add 다 back: 먹 + 이 + 다 → 먹이다. The new verb is transitive — it takes an object (the thing being caused), and the original subject of the base verb becomes that object.

Notice the valency shift: in 아기가 밥을 먹어요 ("the baby eats"), the baby is the eater; in the causative, the baby becomes the one made to eat, and 엄마 is the new subject. The conjugation is ordinary — 먹이다 inflects like any 이다-ending vowel stem: present 먹여요 (meogyeoyo), past 먹였어요 (meogyeosseoyo). See causative valency and case-marking for how the objects line up.

The -이- inventory

-이- attaches mainly to stems ending in a vowel or in ㄱ / ㄲ and a few others where the phonology cooperates. Here are the high-frequency members, with the pronounced surface form in brackets:

Two pronunciation points worth internalizing. In 끓이다 the ㅀ batchim resyllabifies: the ㅎ drops and the ㄹ slides onto the 이, giving [끄리다] — not "kkeulida." In 붙이다 the ㅌ meets 이 and palatalizes to ㅊ: [부치다], the same sound-change you saw in 같이 → gachi.

Several -이- causatives are simply English verbs

This is the reframing English speakers most need. You are tempted to read 먹이다 as "make eat," 죽이다 as "make die," 붙이다 as "make stick" — mechanical, phrase-like glosses. But English already has single words for these causatives: feed, kill, attach. The Korean is exactly as compact as the natural English; the "make V" gloss is a translation artifact, not the meaning.

So don't apply a formula in real time — store 먹이다, 죽이다, 붙이다, 속이다, 줄이다 as vocabulary the way you store any transitive verb. Trying to "compute" them mid-sentence is slower and error-prone.

보이다: causative and passive in one form

보이다 deserves special attention because it leads a double life. From 보다 ("see"), 보이다 is both the causative "show" (cause someone to see) and the passive "be seen / be visible." Only the particles and context tell them apart.

In the first, 보이다 has an agent showing something; in the second, the sea is simply the thing being seen, marked with 가, with no one doing the showing. This causative/passive overlap runs through several suffix verbs and is easy to trip over — the dedicated passive/causative homophone page collects the whole set (보이다, 읽히다, 업히다, 안기다 …). For -이- specifically, keep 보이다's two faces in view.

The hard truth: the suffix is not predictable

Here is where honesty matters more than a tidy rule. You might hope the final consonant of the stem tells you which causative suffix to use. It gives hints — but it does not decide. 먹다 (ㄱ-final) takes 이 → 먹이다, yet 익다 (also ㄱ-final) takes 히 → 익히다. There is no phonological law that forces one over the other; the choice is lexically fixed and memorized per verb. That is why the productive-looking forms below are simply wrong:

• 입다 → 입히다 (not ×입이다) — "dress someone"
• 앉다 → 앉히다 (not ×앉이다) — "seat someone"
• 잡다 → 잡히다 (passive, not ×잡이다)

Do not derive -이- freely. Learn the -이- set as a closed list, and when a verb isn't on it, assume a different suffix and check. The suffix-selection page lays out the tendencies (and their exceptions) in full; the sister pages cover -히- and the rest.

Common Mistakes

1. Deriving -이- productively for a -히- verb. These stems look eligible but are lexically fixed to 히.

2. Same error with 앉다. "Seat someone" is 앉히다.

3. Mispronouncing 끓이다. The ㅀ resyllabifies; it is [끄리다], never "kkeulida."

4. Building a "make eat" phrase instead of 먹이다. Feed is one verb in both languages.

5. Ignoring 보이다's two readings. Watch the particle: an object marked with 을/를 (보여 주다) is the causative "show"; a subject marked with 이/가 (잘 보여요) is the passive "be visible."

Key Takeaways

• -이- slots between stem and ending to form a causative: 먹다 → 먹이다, 죽다 → 죽이다.
• It favors vowel- and ㄱ-final stems, but membership is a memorized list, not a rule — 먹다 takes 이 while the ㄱ-final 익다 takes 히.
• Many -이- causatives are plain English verbs: feed, kill, attach, reduce, deceive, melt — store them as vocabulary.
• Mind the pronunciations: 끓이다 = [끄리다] (ㅀ resyllabifies), 붙이다 = [부치다] (ㅌ palatalizes).
• 보이다 is both causative "show" and passive "be seen" — particles and context disambiguate.