Comparative Stem Changes: -mpi → -mma- | Finnish Grammar

You know how to build a comparative — iso → isompi, pieni → pienempi — from the comparative -mpi page. This page is about what happens when you have to inflect it. A comparative is a full adjective, so it declines in every case and agrees with its noun. The catch is that the shape -mpi you see in the dictionary is only the nominative. The moment you add a case ending, -mp- usually softens to -mm-: isompi → isomman → isommassa. Miss this and you produce *isompissa, a form no Finn writes.

Two stems: strong -mpa- and weak -mma-

Think of a comparative as having two working stems:

a strong stem isompa- (with -mp-), and
a weak stem isomma- (with -mm-).

The switch between them is nothing exotic — it is ordinary consonant gradation, the very same mp → mm assimilation that turns kampa ("comb") into the genitive kamman. The nasal m swallows the p and doubles. In the comparative, this gradation runs automatically as you decline the word.

Here is the full singular and part of the plural for isompi ("bigger"), with the grade marked:

Case	Form	Grade
Nominative sg	isompi	— (base)
Partitive sg	isompaa	strong -mp-
Genitive sg	isomman	weak -mm-
Inessive sg	isommassa	weak -mm-
Elative sg	isommasta	weak -mm-
Illative sg	isompaan	strong -mp-
Adessive sg	isommalla	weak -mm-
Allative sg	isommalle	weak -mm-
Essive sg	isompana	strong -mp-
Translative sg	isommaksi	weak -mm-
Nominative pl	isommat	weak -mm-
Partitive pl	isompia	strong -mp-

The pattern is exactly the gradation rule: the strong -mp- stays in the partitive, illative, essive, and partitive plural; the weak -mm- appears in the genitive, inessive, and the majority of oblique cases. If you already know when a noun like kampa keeps mp versus mm, the comparative follows the identical logic.

The rule of thumb

For everyday use you do not need to memorise the whole table. Two facts carry you:

The genitive and most oblique cases take -mm-: isomman, isommassa, isommalla, isommat.
The partitive singular keeps -mp-: isompaa, not *isommaa.

Muutimme isompaan asuntoon lähemmäs keskustaa.

We moved into a bigger flat closer to the centre. (illative isompaan)

Asumme nyt isommassa asunnossa.

We now live in a bigger flat. (inessive isommassa)

Etsin isompaa reppua tätä matkaa varten.

I'm looking for a bigger backpack for this trip. (partitive isompaa)

That partitive singular is the form that trips up even intermediate learners, precisely because everything around it has softened to -mm-. When the comparative is the object of a verb that wants the partitive, or the predicate of a mass subject, reach for -mpaa/-mpää:

Haluaisin vähän kuumempaa kahvia.

I'd like slightly hotter coffee. (partitive kuumempaa)

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The quickest self-check: the strong -mp- survives in exactly four forms — the partitive singular (-mpaa), the illative (-mpaan), the essive (-mpana), and the partitive plural (-mpia). Every other case gradates to the weak -mm-: genitive -mman, inessive -mmassa, adessive -mmalla, translative -mmaksi, plural -mmat. What ties the four strong forms together is that their ending leaves the -pa-/-pä- syllable open; the essive -mpana does too, which is why it keeps -mp- even though its ending begins with a consonant.

It agrees like any adjective

Once you have the right stem, the comparative behaves like a completely ordinary attributive adjective: it copies the case and number of its noun. The comparative and the noun gradate independently, each by its own rules:

Annoin paikkani vanhemmalle naiselle.

I gave up my seat to an older woman. (allative on both: vanhemmalle naiselle)

Pienemmille lapsille on oma leikkinurkka.

There is a play corner of their own for the younger children. (allative plural: pienemmille lapsille)

Kalliimmasta hotellista oli kauniimmat näkymät.

The more expensive hotel had more beautiful views.

Harmony: -mma- vs -mmä-

The consonant grade is one axis; vowel harmony is the other, and the two are independent. A back-vowel adjective takes -mma- / -mpa-, a front-vowel one takes -mmä- / -mpä-. Compare isompi (back) with pienempi (front):

Case	Back: isompi	Front: pienempi
Genitive sg	isomman	pienemmän
Partitive sg	isompaa	pienempää
Inessive sg	isommassa	pienemmässä
Illative sg	isompaan	pienempään
Partitive pl	isompia	pienempiä

Etsimme pienempää autoa, tämä on liian iso perheellemme.

We're looking for a smaller car, this one is too big for our family. (partitive pienempää)

Common Mistakes

❌ Asumme isompissa asunnossa.

Incorrect — the oblique cases gradate to -mm-: isommassa, not the frozen -mpi.

✅ Asumme isommassa asunnossa.

We live in a bigger flat.

❌ Haluaisin kuumemmaa kahvia.

Incorrect — the partitive singular keeps the strong -mp-: kuumempaa, not *kuumemmaa.

✅ Haluaisin kuumempaa kahvia.

I'd like hotter coffee.

❌ Annoin paikkani vanhempalle naiselle.

Incorrect — the allative is a weak-grade case: vanhemmalle, not *vanhempalle.

✅ Annoin paikkani vanhemmalle naiselle.

I gave up my seat to an older woman.

❌ Etsin pienempaa autoa.

Incorrect — the front-vowel partitive is pienempää with -ä-, not the back -paa.

✅ Etsin pienempää autoa.

I'm looking for a smaller car.

Key Takeaways

The comparative has a strong stem -mpa-/-mpä- and a weak stem -mma-/-mmä-; the switch is ordinary mp → mm gradation.
Most oblique cases take the weak -mm-: isomman, isommassa, isommat.
The partitive singular keeps the strong -mp-: isompaa, pienempää — the classic exception. (So do the illative, essive, and partitive plural.)
The strong -mp- is limited to four forms — partitive sg (-mpaa), illative (-mpaan), essive (-mpana), partitive pl (-mpia); every other case gradates to weak -mm-.
Harmony is separate: back -mma- (isomman) vs front -mmä- (pienemmän).

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