Forming Adverbs with -sti

English turns most adjectives into adverbs by adding -ly: quick → quickly, beautiful → beautifully. Finnish has an almost perfectly parallel tool — the suffix -sti — and once you learn where to attach it, you can build a manner adverb from nearly any adjective. There is exactly one common word that breaks the pattern, and it is the most useful adjective in the language: hyvä (good).

The basic rule: adjective stem + -sti

The key thing to understand is that -sti does not attach to the dictionary form (the nominative). It attaches to the adjective's inflectional stem — the same stem you see when you put an ending on the word. For adjectives that end in -a/-ä or -o/-ö in the dictionary, the stem is identical to the nominative, so nothing surprising happens:

Notice that vowel harmony rides along automatically: a front-vowel adjective like selvä takes -sti with the harmony already fixed by the stem (selvästi), while a back-vowel adjective like huono gives huonosti. You never choose between -sti variants — the suffix is always spelled the same; only the stem in front of it changes. If you are shaky on why selvästi has ä and huonosti has a, review vowel harmony.

Why the stem matters: kaunis → kauniisti

The stem rule becomes visible — and easy to get wrong — with adjectives whose dictionary form differs from their stem. Take kaunis (beautiful). Its inflectional stem is kaunii- (you see it in kauniin, kauniita). Since -sti attaches to that stem, the long ii carries straight into the adverb:

The adverb hitaasti hides two changes at once. First, hidas has consonant gradation — the -d- of the dictionary form is the weak grade, and the stem hitaa- restores the strong -t-. Second, the long aa belongs to the stem. Build the adverb off the nominative and you would produce the non-word *hidasti; build it off the stem and you get the correct hitaasti. If gradation like d ↔ t is unfamiliar, see consonant gradation.

Two more gradation cases worth seeing, because the weak grade of the stem surfaces in the adverb:

Here helppo (easy, stem helpo-, weak grade pp → p) gives helposti, and tarkka (precise, stem tarka-, weak grade kk → k) gives tarkasti.

Adjectives in -inen become -sesti

A very large family of Finnish adjectives ends in -inen (rehellinen honest, ystävällinen kind, tavallinen ordinary, todennäköinen probable). Their stem ends in -se-, so their adverbs all come out in -sesti:

Recognising the -inen → -sesti pattern lets you produce dozens of adverbs on sight, because -inen is one of the most productive adjective endings in the language.

The irregular one: hyvä → hyvin

The single high-frequency exception is hyvä (good). Its adverb is not *hyvästi — it is the suppletive form hyvin (well). This is worth memorising as a fixed pair, side by side with a regular adverb, so the contrast sticks:

One extra bonus from learning hyvin: it does double duty. Besides "well," it is also the everyday word for "very" — so hyvin hyvä is "very good," hyvin kaunis is "very beautiful." Finnish uses the same manner adverb as a degree intensifier, something English keeps strictly apart (well vs very). The degree use is covered on degree adverbs.

Manner, not degree

One clean insight that keeps -sti adverbs in their lane: -sti marks manner — how an action is done — and nothing else. It does not make comparatives ("more quickly" is a separate form, see comparing adverbs) and it is not the way to say "very." When you want how something happens, reach for -sti; when you want how much, that is a different set of words.

Common Mistakes

Key Takeaways

• Add -sti to the adjective's inflectional stem, not the dictionary form: nopea → nopeasti, hidas → hitaasti, kaunis → kauniisti.
• Stem changes — consonant gradation and vowel length — carry into the adverb: helppo → helposti, tarkka → tarkasti, kaunis → kauniisti.
• Adjectives in -inen give adverbs in -sesti: ystävällinen → ystävällisesti.
• Memorise the one irregular: hyvä → hyvin (never *hyvästi, which means "goodbye"). And remember hyvin also means "very."