Standard written Finnish often uses possessive suffixes instead of or in addition to genitive forms of personal pronouns. For example, “my car” is autoni in literary Finnish; it consists of the word auto (car) and the suffix of the first person singular, ni.
It is possible to use additionally a personal pronoun, corresponding to English “my”: minun autoni. However, in literary style, this should be used only for emphasis, e.g. when referring to my car in explicit comparison with your car.
No possessive suffix is used when the genitive form of personal pronoun is used in addition to a noun in the genitive form, connected with a word like ja (and) or tai (or). Thus, although we write minun autoni, we write minun ja Hannan auto or Hannan ja minun auto when referring to a car owned by me and Hanna. Similarly, when two pronouns are used, no possessive suffix appears, e.g. sinun ja minun auto.
In the third person, a personal pronoun in the genitive is normally used in addition to a possessive suffix, e.g. hänen autonsa (his/her car). If the suffix relates to the subject person of the statement, the personal pronoun is omitted, however. Thus, we can say Myin hänen autonsa (I sold his/her car), but Hän myi autonsa (He/she sold his/her car). You might also see usage like Matti ja ystävänsä Ville, but this is substandard: the pronoun hänen must not be omitted here: Matti ja hänen ystävänsä Ville (Matti and his friend Ville).
The third person suffixes are also used as corresponding to English words “its” and “their” when they refer to the subject of the clause. In this context, they are never preceded by the genitive of a pronoun, sen or niiden. For example, “The company fired its CEO” would be Yhtiö erotti toimitusjohtajansa. But as the subject of a statement, “its CEO” is sen toimitusjohtaja.
Third person possessive suffixes are not used when the noun is preceded by a genitive attribute other than third person personal pronoun hänen or heidän. Thus, we say hänen autonsa (his/her car) but Liisan auto (Liisa’s car).
The meanings of possessive suffixes vary, partly like the meanings of English words “my”, “your” etc. vary, but also because the suffixes have special idiomatic use.
The term “possessive suffix” is commonly used in Finnish grammars, but it is somewhat misleading, since these suffixes mostly indicate something else than possession. The Finnish term omistusliite is even more misleading, as it refers to ownership. Possessive suffixes would perhaps best be described as personal suffixes of nouns.
These suffixes indicate possession rather rarely; autoni usually indicates possession of some kind, and jalkani (my leg) may be interpreted as possessive in an abstract sense, but elämäni (my life), veljeni (my brother), matkani (my trip), or vastustajani (my opponent) are hardly possessive expressions.
A possessive suffix is really a relational suffix, relating a word to a person or a thing. In many phrases, a possessive suffix has no identifiable meaning: it is just part of a grammatical structure.
Possessive suffixes are attached to nouns, but usually not to adjectives. For example, we say vanha autoni (my old car) without adding the ni suffix to the adjective vanha. However, an adjective used as a noun can have a possessive suffix, e.g. meidän nuoremme (our young folks), using nuori (young) as a noun (young person).
Exceptionally, adjectives that are used to express equality comparison of a property can take possessive suffixes. For example, the adjective pituinen (from pituus “length, tallness”) is used in expressions like Paavon pituinen mies (a man as tall as Paavo), and when the reference is expressed with a personal pronoun, a possessive suffix is used, too, e.g. minun pituiseni mies (a man as tall as I).
This also means that congruence does not apply to possessive suffixes like it applies to case suffixes. For example, if we inflect the expression rakas vanha ystävä in the allative (-lle case), all words get the case suffix: rakkaalle vanhalle ystävälle. However, if a possessive suffix is added, it is appended to the last word only, e.g. rakkaalle vanhalle ystävälleni.
Possessive suffixes are mostly not used with pronouns, except the following:
Possessive suffixes are also used for many participles and infinitives of verbs. In that context, they indicate the agent of the action expressed by the verb. For example, the word kirjoittamani is the past participle form of kirjoittaa (to write), with the 1st person singular suffix ni, and means “which I have written”. Thus, in a clause equivalent like tekemäni työ (work that I have done), the suffix ni indicates the agent of action, not possession of any kind.
The following verb forms may have a possessive suffix:
Many adverb-like words have a possessive suffix that varies according to the person form of the clause. The word mielellään (willingly, with pleasure; preferably) is, by its form, the adessive of mieli (mind) with a possessive suffix, so literally it means “with his/her mind”, but it has a specialized meaning. In standard language, the possessive suffix varies, e.g. Tein sen mielelläni (I did it with pleasure), Me teimme sen mielellämme (We did it with pleasure).
In such usage, the variation of the possessive suffix has no meaning, since it simply agrees with the subject. This phenomenon can be classified as congruence.
It is increasingly common, though still substandard, to use invariably the third person form mielellään, e.g. Tein sen mielellään. This means that such words are used purely as adverbs.
Most postpositions can have possessive suffixes. This is natural, because many of them are originally case forms of nouns. For example, we say auton edessä (in front of a car) but edessäni or minun edessäni (in front of me). In common spoken language, however, possessive suffixes are mostly not used even in such expressions; instead, we say mun edessä.
Possessive suffixes can be used for all case forms except the instructive. However, they are very rarely used for abessive forms of nouns, e.g. teoittamme (without our deeds). On the other hand, certain infinitives in the instructive form often take the abessive suffix and a possessive suffix (indicating the agent), e.g. tietä|mä|ttä|ni (without my knowing).
Many sayings contain possessive suffixes, especially third person suffixes. For example, the saying se ottaa aikansa (it takes its time) has the suffix nsa as an integral part, and mennä menojaan (to go off; to run its course), with the an suffix, is heavily idiomatic (literally “to go his/her/its goings”). In the latter phrase, the possessive suffix varies by the subject person, e.g. Menin menojani (I went off).
In sayings like Mikä hän on miehiään? (What kind of a man is he?, literally “what he is of his men”), the possessive suffix is part of the phrase with no special meaning. However, it still takes the person form corresponding to the subject, e.g. Kuka sinä olet miehiäsi?
There are also adverbs where the possessive suffix does not really relate to anything and is invariably in a 3rd person form, e.g.