Plural Stem Changes: talo → taloi-, poika → poji- | Finnish Grammar

You already know that oblique plurals are built on stem + -i- (talo → taloissa). The catch is that the -i- rarely sits politely next to the stem vowel — the two collide, and the stem vowel reacts. Sometimes it survives (talo → taloi-), sometimes it turns into something else (kissa → kissoi-), and sometimes it vanishes (koira → koiri-). These changes are not memorized word by word: they follow rules keyed to the stem vowel and, for the -a stems, to the vowel of the first syllable. This page gives you those rules. It is the difference between reciting plurals and generating them.

The master table

Here is every outcome for a two-syllable stem, with a safe example for each. Read the "Before -i-" column as "what happens to the final stem vowel when -i- is added."

Final stem vowel	Before -i-	Example (stem → plural stem → inessive pl)
o, u, ö, y	kept	talo → taloi- → taloissa; koulu → kouluissa
-a, first syllable a / e / i	→ o	kala → kaloi- → kaloissa; kissa → kissoi- → kissoissa
-a, first syllable o / u	dropped	koira → koiri- → koirissa; muna → muni- → munissa
-ä (two-syllable)	dropped	kylä → kyli- → kylissä; päivä → päivi- → päivissä
-e	dropped	kive- → kivi- → kivissä; järve- → järvi- → järvissä
long vowel (aa, uu…)	shortens	maa → mai- → maissa; puu → pui- → puissa
ie, uo, yö	high vowel drops	työ → töi- → töissä; tie → tei- → teissä

The rest of this page unpacks the two rows that cause almost all the trouble: the -a stems, and the changes that come tangled up with gradation.

The heart of it: -a → o versus -a dropped

This is the rule most learners never quite pin down, so here it is baldly. For a two-syllable stem ending in -a, the plural -i- does one of two things, and the vowel of the first syllable decides which:

First syllable has a, e, or i → the final -a becomes -o.
First syllable has o or u → the final -a is dropped.

Word	First vowel	Plural stem	Inessive pl
kala (fish)	a	kaloi-	kaloissa
sana (word)	a	sanoi-	sanoissa
kissa (cat)	i	kissoi-	kissoissa
koira (dog)	o	koiri-	koirissa
muna (egg)	u	muni-	munissa

Suomalaisissa saduissa on usein karhuja ja kettuja.

Finnish fairy tales often have bears and foxes in them.

Kaloissa on paljon hyviä rasvahappoja.

Fish contains a lot of good fatty acids.

Koirissa on eroja: toinen on rauhallinen, toinen vilkas.

There are differences among dogs: one is calm, the other lively.

💡

When you meet a new two-syllable -a word, look at its first vowel, not the whole word. Rounded (o, u) → drop the -a; open or front-ish (a, e, i) → turn it into -o. This is a genuine phonological rule, so it works on words you have never inflected before — that is exactly what makes it worth learning as a rule rather than as a list.

Honesty check: this rule covers the overwhelming majority of two-syllable -a words, but Finnish has a small set of exceptions where usage simply has to be learned (and longer words behave differently again — see type classes). The -ä stems are simpler: in two-syllable words the -ä is always dropped, never turned into -ö (kylä → kylissä, päivä → päivissä).

When gradation piles on top: poika → poji-

The vowel change and consonant gradation are independent processes, and they can strike the same word at once. The showpiece is poika ("boy"):

poika has o in the first syllable, so the final -a is dropped: poik- + i-.
In the closed syllable of the oblique plural, the k takes its weak grade — and for this word the weak grade is k → j (an irregular change, shared with aika "time"): poj- + i-.

The result is the plural stem poji-: pojissa (in the boys — figuratively "among the boys"), pojilla, pojille.

Case	Form	Grade
nominative pl	pojat	weak (poj-)
inessive pl	pojissa	weak (poj-)
adessive pl	pojilla	weak (poj-)
partitive pl	poikia	strong (poik-)
genitive pl	poikien	strong (poik-)

Note the split inside the plural itself: the local cases (pojissa, pojilla) show the weak grade because their syllable is closed, but the partitive and genitive (poikia, poikien) keep the strong grade in an open syllable. The mechanics of k → j / ∅ are on the k → zero gradation page.

Naapurin pojilla on aina jokin peli kesken.

The neighbour's boys always have some game going on.

Puhuin eilen molemmille pojille erikseen.

I spoke to both boys separately yesterday.

Long vowels and diphthongs

A long vowel cannot stand before -i-, so it shortens — one of the two identical vowels drops out: maa → mai- (maissa, "in the countries"), puu → pui- (puissa, "in the trees"), suu → sui-.

An opening diphthong (ie, uo, yö) loses its high vowel (i, u, y), leaving the mid vowel plus -i-: tie → tei-, suo → soi-, työ → töi-. This is why "at work" is töissä, not *työissä.

Olen töissä kello viiteen asti.

I'm at work until five o'clock.

Monissa maissa juna on halvempi kuin lento.

In many countries the train is cheaper than the plane.

Linnut pesivät noissa vanhoissa puissa.

The birds nest in those old trees.

The -i-final trap: two words that look alike

Finally, the corner that catches even advanced learners. Nouns ending in -i split into two groups that behave in opposite ways before the plural -i-:

Old, native words whose real stem ends in -e (ovi → ove-): the -e is dropped. Plural stem ovi-, so ovissa — which coincidentally looks unchanged.
New or foreign words with a genuine -i stem (lasi, paperi, posti): the -i changes to -e. Plural stem lasei-, paperei-, so laseissa, papereissa.

Word	Real stem	Plural stem	Inessive pl
ovi (door)	ove-	ovi- (e dropped)	ovissa
järvi (lake)	järve-	järvi- (e dropped)	järvissä
lasi (glass)	lasi-	lasei- (i → e)	laseissa
paperi (paper)	paperi-	paperei- (i → e)	papereissa

There is no shortcut for telling these apart from the -i alone — you have to know whether the word is an old -e stem or a genuine -i word, and that is usually a matter of age and origin (native versus loan). This is one of the genuinely hard, partly-lexical corners of the plural, so flag it and learn such words with a plural form attached.

Vieraat toivat kukkia, ja panin ne isoihin laseihin.

The guests brought flowers, and I put them in big glasses.

Kesämökki on kahden järven välissä.

The summer cottage is between two lakes.

Common Mistakes

❌ Kissa nukkuu monissa paikoissa, jopa kirjaissa.

Incorrect — kirja has i in the first syllable, so -a → o: kirjoissa, not *kirjaissa.

✅ Kissa nukkuu monissa paikoissa, jopa kirjoissa.

The cat sleeps in many places, even in/on the books.

❌ Vietimme kesän monissa kyläissä.

Incorrect — a two-syllable -ä drops before -i-, it never becomes -ö or stays: kylissä.

✅ Vietimme kesän monissa kylissä.

We spent the summer in many villages.

❌ Naapurin poikissa on kova meteli.

Incorrect — with the local case, poika is weak: pojissa (k → j), not *poikissa.

✅ Naapurin pojissa on kova meteli.

There's a lot of noise coming from the neighbour's boys.

❌ Tarjoilin mehua lasissa kaikille vieraille.

Incorrect if you mean several glasses — lasissa is 'in the glass' (singular). lasi is an i → e word, so the plural is laseissa.

✅ Tarjoilin mehua laseissa kaikille vieraille.

I served juice in glasses to all the guests.

Key Takeaways

The plural -i- fuses with the stem vowel; the change is rule-governed, not memorized per word.
Two-syllable -a: → o if the first vowel is a/e/i (kissa → kissoi-), dropped if it is o/u (koira → koiri-). Two-syllable -ä is always dropped (kylä → kylissä).
-e drops (kive- → kivi-); long vowels shorten (maa → mai-); ie/uo/yö lose the high vowel (työ → töi-).
Gradation can co-occur with the vowel change: poika → poji- (a dropped and k → j), with the strong grade surviving in poikia, poikien.
The -i-final trap: old -e stems drop the e (ovi → ovissa), true -i words shift i → e (lasi → laseissa) — a partly lexical distinction worth flagging.

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