A huge slice of everyday Finnish vocabulary ends in -us or -ys: vastaus (answer), kysymys (question), rakennus (building), kokous (meeting), ostos (a purchase). These are mostly deverbal nouns — the noun made from a verb, naming the act or its result. On the surface they end in a consonant, which Finnish words normally avoid, and the reason is that the -s is the tip of a hidden stem in -ukse-/-ykse-. Get that stem and the whole paradigm falls out: vastaus → vastauksen, vastausta, vastauksia.
This page has a twin problem you must meet head-on: the abstract quality nouns like rakkaus (love) and kauneus (beauty) also end in -us but hide a different stem, -uude-. We will draw the line clearly and send you to the -uus/-yys page for the other class.
The stem: -us → -ukse-, -ys → -ykse-
Historically the ending was \-ukse, and the final *ks simplified to -s only in the bare nominative. Everywhere else the full -kse-* comes back:
| Case | vastaus (answer) | kysymys (question) |
|---|---|---|
| Nominative | vastaus | kysymys |
| Genitive | vastauksen | kysymyksen |
| Partitive | vastausta | kysymystä |
| Illative | vastaukseen | kysymykseen |
| Inessive | vastauksessa | kysymyksessä |
| Nom. plural | vastaukset | kysymykset |
| Part. plural | vastauksia | kysymyksiä |
Vowel harmony does all the front/back switching for you: back-harmony words take -ukse- (vastaus), front-harmony words take -ykse- (kysymys). Note that kysymys is front throughout (-mys, -myksen, -mystä, -myksiä) because y and ä are front vowels.
En saanut vastausta kysymykseeni.
I didn't get an answer to my question.
Kaikki vastaukset ovat oikein.
All the answers are correct.
Kokouksessa esitettiin monta hyvää kysymystä.
Many good questions were raised at the meeting.
These are action and result nouns
Almost every word in this class is built from a verb, and that is the surest way to recognize it. If you can peel off a verb, expect the -ukse-/-ykse- stem:
| Verb | Noun (-us/-ys) | Genitive (-ukse-/-ykse-) |
|---|---|---|
| vastata (to answer) | vastaus | vastauksen |
| kysyä (to ask) | kysymys | kysymyksen |
| rakentaa (to build) | rakennus | rakennuksen |
| harjoitella (to practise) | harjoitus | harjoituksen |
| ostaa (to buy) | ostos | ostoksen |
| ehdottaa (to suggest) | ehdotus | ehdotuksen |
Kaupungin uusi rakennus valmistuu ensi keväänä.
The city's new building will be finished next spring.
Tänään on kielioppiharjoitus kello kolme.
There's a grammar exercise today at three o'clock.
Kävin ostoksilla ja unohdin puolet ostoksista kotiin.
I went shopping and left half of the purchases at home.
The partitive is the giveaway: -sta/-stä, not -tta/-ttä
The single most useful diagnostic against the -uus abstracts is the partitive singular. This class keeps its -s and just adds -ta/-tä, giving a single t: vastaus + ta → vastausta, kysymys + tä → kysymystä. The abstract class doubles the t: kauneutta, rakkautta. So:
| This class (deverbal) | The other class (abstract) | |
|---|---|---|
| Partitive | vastausta, kysymystä | kauneutta, rakkautta |
| Genitive | vastauksen, kysymyksen | kauneuden, rakkauden |
Odotan vielä lopullista vastausta.
I'm still waiting for a final answer.
English has no equivalent hidden stem
English builds nouns from verbs too — build → building, answer → answer — but the noun never grows a secret extra syllable. In Finnish the noun rakennus carries the invisible -ukse- that only appears once you inflect it, so a learner who reasons "it ends in -us, so I'll just add endings to -us" produces *rakennusin instead of rakennuksen. There is no shortcut except to store the word together with its genitive from the very first time you meet it.
Yritys teki asiakkaalle houkuttelevan tarjouksen.
The company made the customer an attractive offer.
Lääkäri kirjoitti lähetteen tutkimusta varten.
The doctor wrote a referral for an examination.
Where the -us ending fools you
Here is the honest difficulty. The single-vowel ending -us/-ys is usually this deverbal class, but a real minority of -us words are abstract quality nouns that take the -uude- stem instead — and you cannot always tell from the spelling:
- vastaus (answer) → vastauksen — deverbal, this class.
- rakkaus (love) → rakkauden — abstract quality, the other class.
Both end in -aus, yet they decline differently. What separates them is meaning and derivation, not the letters: vastaus is "the act/result of answering" (from a verb), while rakkaus is "the quality of being dear" (from the adjective rakas). A reliable rule of thumb: a doubled vowel before the -s (-uus, -yys: mahdollisuus, ystävyys) is always the abstract class; a single vowel (-us, -ys) is usually this deverbal class but occasionally abstract (kauneus, vapaus, rakkaus, terveys). When in doubt, check whether the word names an abstract quality — if it does, expect -uude-.
Common Mistakes
❌ En saanut yhtään vastaus.
Incorrect — 'not a single answer' needs the partitive off the -s stem: vastausta.
✅ En saanut yhtään vastausta.
I didn't get a single answer.
❌ Tämän rakennusin katto vuotaa.
Incorrect — the genitive is not *rakennusin; the stem expands to -ukse-: rakennuksen.
✅ Tämän rakennuksen katto vuotaa.
This building's roof is leaking.
❌ Minulla on pari kysymysta.
Incorrect — kysymys is front-harmony throughout, so the partitive is kysymystä with ä, not kysymysta.
✅ Minulla on pari kysymystä.
I have a couple of questions.
❌ Kaikki ostoit ovat kassissa.
Incorrect — the plural of ostos is ostokset, from the -ukse- stem, not an invented *ostoit.
✅ Kaikki ostokset ovat kassissa.
All the purchases are in the bag.
Key Takeaways
- -us/-ys deverbal nouns hide the stem -ukse-/-ykse-: vastaus → vastauksen, vastausta, vastauksia.
- These name an action or its result and almost always come from a verb — finding the verb confirms the class.
- The partitive is the diagnostic: single-t -sta/-stä here (vastausta), doubled-t -tta/-ttä in the abstract class (rakkautta).
- A doubled vowel (-uus/-yys) is always the abstract class; a single vowel is usually this one — but kauneus, rakkaus, vapaus are single-vowel abstracts, so check the meaning.
- Store each new -us word together with its genitive so the hidden -ukse- never surprises you.
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Start learning Finnish→Related Topics
- -uus / -yys Nouns: rakkaus → rakkaude-B1 — Abstract quality nouns in -uus/-yys hide an -uude-/-yyde- stem: kauneus → kauneuden, kauneutta. How they differ from the -ukse- nouns.
- Consonant StemsB1 — Many nouns carry a second stem ending in a consonant, used before the partitive singular -ta/-tä and a few genitive plurals: käsi→kät- (kättä), nainen→nais- (naista).
- Abstract Nouns: -us, -uus, -minenB2 — Deriving abstract and event nouns: -us/-ys names an act or result from a verb, -uus/-yys a quality from an adjective, and -minen a verbal noun from any verb.
- Partitive Endings: -a/-ä, -ta/-tä, -tta/-ttäA2 — The partitive has three shapes chosen by the stem: -a/-ä on single-vowel stems, -ta/-tä on consonants and long vowels, -tta/-ttä on -e nouns.
- Noun Type Classes: A Reference MapC1 — A consolidated map of Finnish declension types — nominative, genitive, partitive, and stem for the dozen patterns behind the KOTUS dictionary type numbers.