The Fifteen Cases

Finnish is an agglutinative language, meaning it 'glues' information onto words. In the traditional count of 15 cases, cases can be grouped as follows. Modern descriptions sometimes count 14 nominal cases and reserve accusative for special pronoun forms; this is a difference of analysis, not missing endings:

• Grammatical Cases: These define basic roles like subject and object. They include the Nominative (basic form), Genitive (-n, primarily used to show possession, like English 's, and required for the vast majority of Finnish postpositions, e.g., talon edessä, pöydän alla), and Partitive (-a/-ta/-ä/-tä/-tta/-ttä). The Partitive is used for partial objects, ongoing actions, negative sentences, and crucially, after numbers greater than one (e.g., yksi auto, but kaksi autoa). The Accusative (which looks like the Nominative or Genitive for nouns, but has a -t ending for personal pronouns) is used for total or completed objects.
• Interior Locative Cases: Used for things 'inside' or 'into' something. These are the Inessive (-ssa/-ssä), Elative (-sta/-stä), and Illative (-Vn/-seen/-hVn).
• Exterior Locative Cases: Used for surfaces or 'at' a place. These are the Adessive (-lla/-llä), Ablative (-lta/-ltä), and Allative (-lle).
• State Cases: These describe states or roles, such as the Essive (-na/-nä, 'as a...') or Translative (-ksi, 'becoming...').
• Marginal Cases: These are rarer cases used in specific expressions: the Abessive (-tta/-ttä, 'without'), Instructive (typically used in the plural with -in, 'by means of'), and Comitative (-ine-, 'together with', which is always formed on the plural stem, meaning context is required to distinguish singular from plural, and must be followed by a possessive suffix on the noun, but not on modifying adjectives).

To use these cases, you must follow two main rules: vowel harmony (choosing between -a/-ä, -u/-y, and -o/-ö depending on the vowels in the word) and consonant gradation (changing the stem of the word). The history of consonant gradation is connected with syllable structure, but modern forms cannot be predicted simply by whether the final syllable is open. In the common direct-gradation type, genitive and inessive use weak grade (kukka → kukan, kukassa), while illative retains strong grade (kukkaan) despite ending in n. In inverse-gradation types, the vowel stem can be strong even in genitive and inessive (saapas → saappaan, saappaassa). Learn each stem type and case pattern. Additionally, case endings can attach to a singular stem or a plural stem (which usually features the plural marker -i-).

Neutral Vowels and Marginal Cases
The vowels e and i can occur with either harmony group. In ordinary noncompound Finnish words containing only e/i, endings normally use front vowels: pieni → pienessä. In compounds, choose suffix harmony from the last component; loanwords can have special patterns.

The marginal noun cases are not equally productive in conversation. Speakers often use ilman + partitive instead of noun abessive and kanssa instead of comitative. Established instructives such as jalan and käsin remain everyday words. The common verbal -matta construction should not be confused with rare noun abessives.