If a Finnish noun ends in -in — puhelin (telephone), avain (key), keitin (maker/boiler), soitin (musical instrument) — it almost always names a tool or instrument, and it hides the stem -ime-. The dictionary form ends in -n, which native Finnish words avoid, but the moment you inflect it the -n turns into -m- and a vowel appears: puhelin → puhelimen, puhelimessa, puhelimeen. This is one of the most regular and productive noun classes in the language, so learning one word teaches you a whole semantic field.
The rule: -in → -ime-, and n becomes m
Two things happen when you leave the dictionary form:
- A stem vowel -e- appears (these are e-stems at heart).
- The final -n changes to -m before that -e- — a change of nasal place that you must actually write.
| Case | puhelin (phone) | avain (key) |
|---|---|---|
| Nominative | puhelin | avain |
| Genitive | puhelimen | avaimen |
| Partitive | puhelinta | avainta |
| Illative | puhelimeen | avaimeen |
| Inessive | puhelimessa | avaimessa |
| Adessive | puhelimella | avaimella |
| Nom. plural | puhelimet | avaimet |
| Part. plural | puhelimia | avaimia |
Missä minun puhelimeni on? En löydä sitä mistään.
Where's my phone? I can't find it anywhere.
Soitan sinulle puhelimella heti, kun pääsen kotiin.
I'll call you on the phone as soon as I get home.
Unohdin avaimet sisälle ja ovi lukkiutui.
I left the keys inside and the door locked.
Two exceptions to the m: the partitive and the consonant stem
The -n → -m change happens only before the vowel stem -e-. A handful of forms are built on the consonant stem, which keeps the original -n. The one you meet daily is the partitive, formed with -ta/-tä straight onto the -n: puhelin + ta → puhelinta, avain + ta → avainta.
Ostin uutta puhelinta varten suojakuoren.
I bought a protective case for the new phone.
En löydä avainta, vaikka etsin kaikkialta.
I can't find the key, even though I've looked everywhere.
So the same word shows m in most cases (puhelimen, puhelimessa, puhelimet) but n in the partitive (puhelinta). This is not an irregularity — it is the two-stem system: the vowel stem is puhelime-, the consonant stem is puhelin-.
A productive instrument-forming pattern
Finnish coins these tool names from verbs on demand, which is why the class keeps growing with technology. The suffix -in attaches to a verb and means "a thing for doing X":
| Verb | Instrument (-in) | Genitive (-ime-) |
|---|---|---|
| soittaa (to play) | soitin | soittimen |
| keittää (to boil) | keitin | keittimen |
| istua (to sit) | istuin | istuimen |
| valaista (to illuminate) | valaisin | valaisimen |
| suodattaa (to filter) | suodatin | suodattimen |
Notice soitin → soittimen and keitin → keittimen: the single t of the dictionary form becomes a double tt in the inflected forms. That is ordinary t-gradation running in reverse — the closed final syllable of keitin forces the weak grade t, while the open syllable of keittimen restores the strong tt. Not every -in word gradates — avain and puhelin have no k/p/t to gradate, and istuin's t sits in a protected -st- cluster — but when the stem has an exposed k, p, or t, watch for it.
Bändissä jokainen soittaa jotain soitinta.
In the band everyone plays some instrument.
Kahvinkeitin hajosi, joten keitän kahvin pannulla.
The coffee maker broke, so I make the coffee in a pot.
Tämä istuin on liian matala minulle.
This seat is too low for me.
The near-twin: sydän
One extremely common word behaves exactly like the -in class without ending in -in: sydän (heart). Its vowel stem is sydäme- (with the same n → m), and its consonant stem sydän- keeps the n for the partitive:
| Case | Form |
|---|---|
| Nominative | sydän |
| Genitive | sydämen |
| Partitive | sydäntä |
| Illative | sydämeen |
| Nom. plural | sydämet |
Hänellä on iso sydän ja pieni ego.
She has a big heart and a small ego.
Sanoit sen suoraan sydämestä.
You said it straight from the heart.
Why English speakers get this wrong
English keeps a noun's shape fixed — phone is phone in phone's and phones. Finnish makes the visible -n a mask over a stem that ends in -me-, so the instinct to "just add the ending to the word I see" produces *puhelinin or *puhelinssa. The fix is a habit: whenever you learn an -in word, learn its genitive in the same breath — puhelin, puhelimen — and the -ime- stem is locked in.
Common Mistakes
❌ Tämä on minun puhelinin numero.
Incorrect — the genitive is puhelimen, with -n changing to -m before the -e- stem.
✅ Tämä on minun puhelimeni numero.
This is my phone number.
❌ Hukkasin avainen eilen.
Incorrect — the total-object (genitive) form is avaimen, with -n changing to -m before the -e- stem, not the invented *avainen.
✅ Hukkasin avaimen eilen.
I lost my key yesterday.
❌ En löydä puhelimta.
Incorrect — the partitive is built on the consonant stem, which keeps -n: puhelinta.
✅ En löydä puhelinta.
I can't find the phone.
❌ Kuuntelen musiikkia sydänellä.
Incorrect — sydän has the vowel stem sydäme-, so the adessive is sydämellä, not *sydänellä.
✅ Kuuntelen musiikkia koko sydämellä.
I listen to music with all my heart.
Key Takeaways
- -in nouns hide the stem -ime-, and the final -n changes to -m: puhelin → puhelimen.
- The partitive and the consonant stem keep the original -n: puhelinta, avainta.
- The class is productive and names tools/instruments made from verbs: soittaa → soitin, keittää → keitin (watch for t → tt gradation).
- sydän (heart) declines like an -in word: stem sydäme-, partitive sydäntä.
- Always store an -in word with its genitive so the -ime- stem is never a surprise.
Now practice Finnish
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Start learning Finnish→Related Topics
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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).
- Two Stems: Dictionary Form vs Inflectional StemA1 — Finnish case endings attach to an inflectional stem, not to the dictionary form — and the genitive reliably reveals that stem (talon→talo-, käden→käde-).
- Genitive -n: Possession and MoreA1 — The genitive -n (talon, käden, Helsingin) marks possession, the singular total object, and the case every postposition demands — attached with weak gradation.
- 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.