A small but very high-frequency group of Finnish nouns ends in -si in the dictionary but has nothing like -si in most of its inflected forms. Vesi (water) gives genitive veden, partitive vettä, illative veteen — where did the -si go, and where did all those ts and *d*s come from? This page shows that the apparent chaos is one clean system: every one of these words is built on a single historical *t, and that t simply wears three costumes depending on the ending. Once you see the t underneath, the paradigm stops being a list to memorize and becomes something you can predict.
One *t, three surface shapes
Long ago these words ended in \-ti. Two sound changes then reshaped the nominative and scattered the *t:
- *ti → si (assibilation): the old *veti became vesi. This is why the dictionary form ends in -si and hides the t completely.
- t → d (consonant gradation): before a closed syllable the t weakens to d.
So the same underlying t surfaces three ways, and which one you get is fully rule-governed:
| Surface shape | Appears in | Example (vesi) |
|---|---|---|
| d | vowel-stem forms with a closed syllable (genitive, inessive, plural -t…) | veden, vedessä, vedet |
| t | the illative and the consonant stem (open syllable) | veteen, vet- |
| doubled t | the partitive (consonant stem + -tä/-ta) | vettä |
| s | the nominative singular and the whole plural stem | vesi, vesiä, vesissä |
The full paradigm of vesi
| Case | Singular | Meaning |
|---|---|---|
| Nominative | vesi | water |
| Genitive | veden | water's / of water |
| Partitive | vettä | (some) water |
| Inessive | vedessä | in the water |
| Illative | veteen | into the water |
| Adessive | vedellä | on/at the water |
| Nom. plural | vedet | waters |
| Part. plural | vesiä | (some) waters |
| Iness. plural | vesissä | in the waters |
Notice the plural: the -i- of the plural sits right where the historical *i was, so the old *ti re-assibilates and the -si- comes back. That is why vesissä (in the waters) has -si- but vedessä (in the water) has -de-.
Voisitko tuoda minulle lasin vettä?
Could you bring me a glass of water?
Meren vesi on tänä kesänä yllättävän lämmintä.
The sea water is surprisingly warm this summer.
Kaivon vedessä oli kummallinen maku.
The well water had a strange taste.
More members: käsi, uusi, susi, vuosi
The same rules run through the whole class. Front-harmony words take -tä/-de-, back-harmony words take -ta/-de-, but the machinery is identical.
| Word | Genitive (d) | Partitive (tt) | Illative (t) |
|---|---|---|---|
| käsi (hand) | käden | kättä | käteen |
| uusi (new) | uuden | uutta | uuteen |
| susi (wolf) | suden | sutta | suteen |
| vuosi (year) | vuoden | vuotta | vuoteen |
| tosi (true) | toden | totta | toteen |
Ojenna kätesi, niin autan sinut ylös.
Give me your hand and I'll help you up.
Ostimme uuden auton viime vuonna.
We bought a new car last year.
Sudet ulvoivat kaukana metsän reunassa.
Wolves were howling far off at the edge of the forest.
Onko tuo todella totta?
Is that really true?
The number words behave the same way, because viisi (five) and kuusi (six) are historically the same shape: viisi → viiden, viittä, viiteen; kuusi → kuuden, kuutta, kuuteen.
Junassa oli vain viisi matkustajaa.
There were only five passengers on the train.
The singular/plural trap: kädessä vs käsissä
This is where English speakers most often slip. The singular oblique forms use the -de-/-te- stem, but the plural oblique forms revert to -si-. So "in my hand" and "in my hands" look deceptively different:
Mitä sinulla on kädessä?
What do you have in your hand?
Minulla on aina kylmä käsissä talvella.
My hands are always cold in winter.
Kädessä (singular) versus käsissä (plural) is not an irregularity — it is exactly the d/s split from the table, applied to number. Keep the two apart and you will never write *käsessä or *kädissä.
The frozen essive vuonna
Vuosi has one form worth memorizing on its own: the essive vuonna, used for calendar years and "this/last/next year." Historically it is vuote- + -na → vuonna (the tn assimilated to nn), and it is the standard way to date anything.
Vuonna 2020 koko maailma muuttui.
In 2020 the whole world changed.
Tänä vuonna aion opetella suomea kunnolla.
This year I'm going to learn Finnish properly.
Not every -si is this type
A word ending in -si is not automatically a t-stem word. Lapsi (child) looks identical on the surface but is an ordinary e-stem: genitive lapsen (not *laden), partitive lasta. Its -si comes from \-psi, not *-ti, so no *d/t ever appears.
Naapurin lapsi aloittaa koulun tänä syksynä.
The neighbour's child starts school this autumn.
The safe test is the genitive: if the dictionary lists X → Xden (with a d), it is a t-stem word (vesi → veden). If it lists X → Xsen (with an s), it belongs to the e-stem group instead (lapsi → lapsen).
Common Mistakes
❌ Juon paljon vesi joka päivä.
Incorrect — 'a lot of water' needs the partitive, and the stem is a t: vettä, not vesi.
✅ Juon paljon vettä joka päivä.
I drink a lot of water every day.
❌ Pidän uusi puhelin kädessä.
Incorrect — a total object and its adjective inflect: the phone is puhelinta/puhelimen and 'new' agrees as uutta/uuden, never the bare uusi.
✅ Pidän uutta puhelinta kädessä.
I'm holding a new phone in my hand.
❌ Minulla on kipuja molemmissa kädissä.
Incorrect — the plural of käsi reverts to -si-: käsissä, not the invented *kädissä.
✅ Minulla on kipuja molemmissa käsissä.
I have pain in both hands.
❌ Muutin Suomeen vuosin 2019.
Incorrect — 'in the year …' uses the frozen essive vuonna, not an invented case.
✅ Muutin Suomeen vuonna 2019.
I moved to Finland in 2019.
Key Takeaways
- These -si words are built on a single historical *t that surfaces as d (veden), t (veteen, vettä), or s (vesi, vesiä).
- The partitive doubles the t off the consonant stem: vettä, kättä, uutta, vuotta.
- Singular oblique uses -de-/-te- (kädessä); plural oblique reverts to -si- (käsissä) — keep them apart.
- Vuosi has the frozen essive vuonna for dating years.
- A final -si is not proof of this class: lapsi → lapsen is an ordinary e-stem. Check the genitive.
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- 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).
- Qualitative Gradation: t → dA2 — Strong t softens to d in a closed syllable — katu→kadun, äiti→äidin, pöytä→pöydän, tietää→tiedän — a sound that barely exists outside gradation.
- Forming the Illative: The Trickiest EndingB2 — The full illative algorithm: double the vowel (taloon), add -hVn (maahan) or -seen (huoneeseen), plus the plural -ihin/-siin — Finnish's trickiest ending.