The -이 / -히 Adverb Suffixes (and 깨끗이 vs 깨끗히)

Alongside the productive -게 suffix, Korean has a second, much older way of making adverbs: -이 and -히. These generate a closed set of extremely common adverbs — 많다 → 많이 ("a lot"), 조용하다 → 조용히 ("quietly"), 깨끗하다 → 깨끗이 ("cleanly"), 정확하다 → 정확히 ("exactly"). The catch is that, unlike -게, this suffix is not a live rule. You cannot derive a new -이/-히 adverb on the fly; you memorize the ones that exist. And then there's the spelling — whether a given word takes -이 or -히 is a genuine trap that trips up native writers, not just learners.

A frozen list, not a rule

Treat -이/-히 adverbs the way you treat English "well" (from "good"): as irregular vocabulary. There is no productive machinery here — each of these is a fixed dictionary word.

Two of these are worth a phonetic note, because they hide a sound change. 같이 ("together") and 굳이 ("insisting, needlessly") undergo palatalization: the ㅌ/ㄷ before -이 becomes a ㅊ/ㅈ sound, so 같이 is pronounced [가치] gachi and 굳이 is [구지] guji — not [가티] or [구디]. And 많이 is [마니] mani: the ㅎ drops out entirely.

The high-frequency list worth knowing

Because these are vocabulary, it helps to see the common ones gathered in one place. This is not an exhaustive list, but it covers the -이/-히 adverbs you'll actually meet:

Notice how many of them anchor a fixed expression you'll hear constantly: 굳이? ("do I really have to?"), 가만히 있어 ("hold still"), 솔직히 말하면 ("to be honest"). Learning the adverb and its home phrase together is far more efficient than learning it in isolation.

Where -이 and -히 come from

A little history explains why there are two suffixes at all — and why the spelling can't simply follow the sound. Both descend from old adverb-forming suffixes, and the 히 form carries a fossilized trace of 하- (the 하다 of 조용하다, 정확하다). That's the deep reason the rule keys on whether the base is a 하다-word: a 하다-root historically fed the -히 suffix, so 정확하다 → 정확히, 조용하다 → 조용히. Roots that never took 하다 fell to the plain -이 suffix instead: 많다 → 많이, 같다 → 같이.

The complication is that centuries of sound change flattened the two suffixes together in many words, so that today 많이 and 솔직히 rhyme to the ear even though one is -이 and the other -히. Spelling preserves the older distinction that pronunciation lost — which is exactly the situation described on the spelling vs pronunciation page, and exactly why you can't spell these by sound alone.

The spelling: -이 or -히?

Here is the part even Koreans get wrong. 깨끗이 or 깨끗히? 정확이 or 정확히? The two suffixes are near-impossible to tell apart by ear in many words, so spelling is decided by a rule from the official orthography (한글 맞춤법 §51), not by pronunciation. The practical version is this:

So 깨끗이 is correct and 깨끗히 is the classic misspelling: 깨끗하다's root ends in ㅅ, which forces -이. Contrast 정확하다, whose root ends in ㄱ — that takes -히, giving 정확히.

This is honestly one of the harder spelling points in Korean, and there's no elegant shortcut that covers every word — 곰곰이, 더욱이, 일찍이 all take 이 for reasons beyond the two main rules, and you'll simply learn them as you meet them. What you can do reliably is never write ×깨끗히 and never write ×정확이. For why spelling and pronunciation part ways here at all, see spelling vs pronunciation.

Common Mistakes

1. Writing ×깨끗히. The ㅅ-final root forces -이.

2. Writing ×정확이. A 하다-root not ending in ㅅ takes -히.

3. Over-generating a nonexistent -이/-히 adverb. -이/-히 is a closed set; a stem that only takes -게 has no -이/-히 form.

4. Writing ×조용이 (wrong direction). Not ㅅ-final and it takes 하다, so it's -히.

Key Takeaways

• -이/-히 generate a closed, frozen set of high-frequency adverbs (많이, 같이, 조용히, 정확히, 깨끗이) — memorize them, don't derive them.
• The productive rule is -게; -이/-히 is vocabulary, like English "well."
• Spelling by rule: ㅅ-final root → 이 (깨끗이, 느긋이), other 하다-root → 히 (정확히, 분명히, 급히), non-하다 root → 이 (많이, 같이, 굳이).
• So 깨끗이 ✓ / ×깨끗히, and 정확히 ✓ / ×정확이 — the two errors even natives make.