-ni/-si/-nsa

Finnish possessive suffixes indicate who something belongs to. The 3rd person suffixes (-nsa/-nsä and the short form -Vn) follow vowel harmony; the others are invariant.

• -ni: my
• -si: your (singular)
• -mme: our
• -nne: your (plural/formal)
• -nsa / -nsä: his, her, their

Key Rules:
1. The n-drop and t-drop: If a case ending ends in -n (such as the genitive, singular accusative, genitive plural, or illative plural), the -n is dropped before the suffix (e.g., talojen -> talojeni, taloihin -> taloihini). Similarly, the nominative and accusative plural -t is dropped. This means taloni can mean 'my house' (nominative or accusative singular), 'my houses' (nominative or accusative plural), and 'my house's' (genitive singular).

2. Consonant Gradation: The possessive suffixes themselves do not trigger the weak grade. However, if the case ending triggers the weak grade (e.g., the inessive -ssa in hatussa), the stem remains weak when the possessive suffix is added (hatussani, not hattussani). The strong grade only 'returns' in cases like the genitive or accusative singular because the syllable-closing -n is dropped before the possessive suffix (e.g., hatun -> hattuni).

3. Consonant-Final Nouns: For nouns that end in a consonant in their basic form, possessive suffixes attach to the inflected vowel stem (e.g., puhelin -> puhelimeni).

4. Translative Case Exception: When a possessive suffix is added to the translative case, the -ksi ending changes to -kse- (e.g., taloksi -> talokseni).

5. 3rd Person Pronouns: While 1st and 2nd person pronouns (minun, sinun) are optional, 3rd person pronouns (hänen, heidän) must be used unless the possession is reflexive (referring back to the subject).

6. 3rd Person Short Form: In eligible cases, the final short vowel lengthens before -n: talossa → talossaan, alongside talossansa. This alternative is unavailable in nominative forms and in genitives or illatives ending in -n: use hänen talonsa, hänen talojensa, hänen taloonsa. It is also unavailable in singular partitives of a/ä-final bases with a long final vowel: hänen koiraansa. Other partitives allow it: hänen taloaan, hänen kirjojaan. Availability depends on case and word type, not merely the last visible vowel.

7. Word Order: The suffix follows the case ending but comes before particles like -kin. Example: talo-ssa-ni-kin (also in my house).

8. Comitative Case: A noun in the comitative case (-ne-) normally requires a possessive suffix; agreeing adjective modifiers do not, even if possession isn't being emphasized (e.g., vaimoineen 'with his/her/their wife').

Note on Spoken Finnish: In everyday spoken Finnish, possessive suffixes are often dropped entirely, and only the genitive pronoun is used (e.g., mun talo instead of minun taloni).

Retain Partitive Marking
Partitive endings remain before a possessive suffix: kirjaa + -ni → kirjaani, taloa + -ni → taloani, kirjoja + -ni → kirjojani. Compare Luen kirjaani (I am reading my book) with total-object Luen kirjani loppuun (I will finish reading my book). The possessive suffix identifies the owner without removing the object-case contrast.