Possessive Suffix in Careful Style

Possessive suffixes are a hallmark of Standard Language (yleiskieli). In formal contexts, omitting them is a grammatical error.

1. Morphology and Stacking:
The suffix attaches to the inflected vowel stem (e.g., puhelin -> puhelime- + -ni = puhelimeni). While the suffix generally follows the case ending (Stem + Case + Suffix, e.g., talo + -ssa + -ni = talossani), the final case-marking -n or plural -t is dropped in the genitive, nominative plural, and illative before the suffix. Note that adjectives modifying the noun take the case ending but do not take the possessive suffix (e.g., uudessa talossani).

2. Consonant Gradation:
Possessive suffixes attach directly to the strong vowel stem and do not trigger consonant gradation themselves. For example, adding the suffix to the nominative stem maintains the strong grade (e.g., puku -> pukuni). If a case ending (like the inessive -ssa) has already triggered the weak grade, the possessive suffix simply attaches to that weak stem without changing it (e.g., katu -> kadussa -> kadussasi).

3. The 'n-dropping' Rule and Partitive:
When a suffix is added, the final -n of the genitive (both singular and plural) and the -t of the nominative plural are removed. In the illative (both singular and plural), the final -n is dropped before the suffix. The partitive ending remains intact.

• koiran (dog's) + -ni -> koirani
• koirat (dogs) + -ni -> koirani
• koirien (dogs') + -ni -> koirieni
• taloon (into the house) + -si -> taloosi
• taloihin (into the houses) + -ni -> taloihini
• kirjaa (book, partitive) + -ni -> kirjaani

4. The 3rd Person Suffix and Reflexive Distinction:
The 3rd person suffix has two forms: -Vn (vowel lengthening + n) is common after case endings ending in a short vowel, alongside full -nsa/-nsä, and -nsa/-nsä after long vowels and in nominative/genitive forms; these suffixes attach to a vowel, not directly to a consonant. The meaning depends on the genitive pronoun:

• Reflexive (One's own): Use the suffix alone. Hän otti kirjansa. (He took his own book.)
• Non-reflexive (Someone else's): Use the genitive pronoun + the suffix. Hän otti hänen kirjansa. (He took his [someone else's] book.)

5. Non-Possessive Uses:
These suffixes also appear in specific grammatical structures like the final construction (syödäkseni - in order for me to eat), the temporal construction (tultuani - after I came), postpositions (e.g., kanssani - with me), and the agent participle (e.g., ostamani auto - the car I bought).

6. Spoken vs. Written Finnish:
In everyday spoken Finnish (puhekieli), possessive suffixes are often dropped from ordinary noun phrases. Possession can then be expressed with the genitive pronoun (e.g., mun auto instead of minun autoni). Suffixes also survive in spoken fixed expressions and in other grammatical constructions; their use varies by speaker and context.

Stem and Person Checks
Do not merely remove the last letter from a weak genitive or plural form: puvun / puvut → pukuni restores the strong vowel stem before -ni. For first and second persons, the genitive pronoun is optional: autoni or minun autoni. Choose the harmonic third-person ending: talonsa with back vowels, kätensä with front vowels, and nimensä with only neutral e/i. Compare talossaan / talossansa; both suffix variants are possible there.