Superlative Stem Changes: -in → -imma-

The superlative ends in -in in the dictionary form — isoin ("biggest"), pienin ("smallest"), suurin ("largest"). But -in is only the nominative. The moment you put the superlative into any other case — and you must, because a superlative adjective still agrees with its noun — the ending transforms into a two-syllable stem -imma- (back) or -immä- (front). "In the biggest house" is isoimmassa talossa, not *isoinssa talossa. This page is about that transformation, which is exactly parallel to the comparative's -mpi → -mma- one degree lower.

The nominative -in is a mask

Think of -in as a mask worn only in the nominative singular. Underneath sits the real inflectional stem -imma-/-immä-, and every case ending except the partitive attaches to that. Here is iso ("big") run through the common cases:

Notice that the adjective and its noun both inflect: isoimmassa talossa, pienimmälle lapselle. The superlative is not frozen — it declines like any other adjective, just on the -imma- stem.

The partitive is the exception: -inta, not -imma-

There is exactly one place where the -in does not melt into -imma-: the partitive singular. There the ending is -inta / -intä, built straight on the nominative-looking -in-:

The hidden gradation: -imma- ~ -impa-

Here is the point most textbooks skip. The stem is really -impa-/-impä- with strong mp, and it undergoes ordinary consonant gradation (mp ~ mm). In a closed syllable it weakens to mm — that is the -imma- you see in the genitive, the locatives, and the nominative plural. In an open syllable it stays strong as mp — and that surfaces in the illative singular and the whole partitive/genitive/illative plural:

So the "double m" is the everyday face of the stem, but do not overgeneralize it: the illative isoimpaan and the partitive plural isoimpia keep the strong mp, exactly as the comparative does (isompaan, isompia). It is the same gradation, one degree of comparison higher.

Where the -i- comes from: vowel changes at the join

The -in is really the stem plus an -i- element (the same -i- that builds the plural and the comparative). Where that -i- meets the adjective's stem vowel, predictable changes happen:

• Stem-final -e disappears: suure- → suurin, piene- → pienin.
• Stem-final -a / -ä usually disappears: kylmä- → kylmin, vanha- → vanhin.
• Stem-final -o, -u, -y, -ö survive, and the -i- glides on after them: iso- → isoin, paksu- → paksuin.

Why English speakers get this wrong

English superlatives are rigid: "biggest" is "biggest" whether it is the subject, the object, or the object of a preposition — "in the biggest house," "of the biggest houses." Nothing after "-est" ever changes. So the instinct is to treat isoin the same way and glue case endings straight onto it: *isoinssa, *isoinlle. But -in is a nominative mask, and case endings need the -imma-/-impa- stem underneath it. The second instinct is to regularize the partitive — assuming that if the genitive is isoimman, the partitive must be *isoimmaa. It is not; the partitive keeps the -in- and is isointa.

Common Mistakes

Key Takeaways

• The nominative -in is a mask; the real inflectional stem is -imma- / -immä- (weak grade) ~ -impa- / -impä- (strong grade).
• The partitive singular is the exception: -inta / -intä (isointa, pienintä), built on -in-, showing neither mm nor mp.
• The strong grade mp surfaces in the illative singular (isoimpaan) and the partitive/genitive/illative plural (isoimpia); everywhere else you see weak mm.
• Contrast the two partitives: comparative isompaa vs superlative isointa.
• Harmony is fixed by the stem — back isoimman / isointa, front pienimmän / pienintä — and -mm- never simplifies.