As a general concept, congruence means that the grammatical form of a word is the same as the form of another word, at least in some respect. This need not mean that the suffixes are the same. For example, the expression iso kala (small fish) has the illative form isoon kalaan; the suffixes here are different, on and an, but they both are illative suffixes.
We can express congruence by saying that a word congrues with another word.
Congruence is sometimes called agreement.
An adjective used as an attribute of a noun congrues with the noun. A simple example was given above. If a noun has several adjectives as its attributes, they all congrue with it. For example, uusi punainen auto means “new red car”, and to express “in a/the new car”, we need to put all the words into the inessive case (ssA case): uudessa punaisessa autossa.
Congruence may make expressions sound or look funny in their repetitiveness. On the other hand, it often makes the structure of a sentence easier to see: you can infer that words probably belong to the same expression when they are in the same case form.
Many Indo-European language have congruence to some extent, but English has lost it. Congruence shows little signs of vanishing from Finnish, except for expressions like trade names in marketing texts. When a large shopping center was opened with the name Iso omena, “big apple”, the name was used without congruence, e.g. Iso omenassa, for some time. Due to people’s protests, normal congruence, e.g. Isossa omenassa, became dominant. On the other hand, long trade names of products are nowadays common, and in them, companies tend to inflect only the last word, e.g. Valio rasvaton maito : Valio rasvaton maitoa (instead of rasvatonta maitoa). However, such names are not used outside marketing texts.
There are some exceptions to congruence, i.e. expressions with incongruence. A few adjectives such as pikku (little) are always uninflected, i.e. they always have the same form. For example, we say pikku poika (a little boy), pikku pojalle, pikku pojan etc.; often compound words like pikkupoika are used instead of such expressions, with a slight difference in meaning. The most common uninflected adjectives are: aika, aimo, ensi, eri, joka, kelpo, koko, melko, pikku, tosi, viime. Some of these are also used as nouns, with inflection (e.g. aika : ajan : ajat), and tosi is an inflected adjective when used in the meaning “true”. Colloquially, some nouns are used as uninflected adjectives, e.g. ihme (meaning “miracle” as a noun, “weird” as an adjective).
There are also several sayings where congruence is broken so that an adjective and a noun are both inflected, but in different cases. From hyvä (good) and mieli (mind, state of mind), we can form the expression hyvällä mielellä (in good mood), but we can alternatively use plural forms, and then the parts are in different cases: hyvillä mielin (instead of hyvillä mielillä). The form hyvillä is adessive, the form mielin is instructive plural, having here the same meaning as the adessive. The noun can also be in the partitive case, e.g. pitkän aikaa (for a long time).
In some phrases, incongruence is obligatory, but in others, it varies with a congruent phrase. Additional examples, in alphabetic order by the main word (the noun): vähän aikaa, vähäksi aikaa, sillä aikaa, tällä erää, tällä haavaa, näillä keinoin ∼ keinoilla, samalla kertaa ∼ kerralla, missä kohtaa ∼ kohdin ∼ kohdassa, näillä main ∼ mailla, pahoilla mielin, kaikella muotoa, missä määrin, näiltä nurkin ∼ nurkilta, tältä osin ∼ osalta, toiselle puolen ∼ puolelle, puolelta päivin, näillä main ∼ mailla, näillä seuduin ∼ näillä seuduilla, tällä tapaa ∼ tavoin ∼ tavalla, tällä tietoa, yksissä tuumin, sillä välin.
Such expressions may have incongruence in number, too. If the main word is in the instructive, it is plural, as the instructive almost always is, but the attribute can be singular, e.g. tällä keinoin as an alternative to näillä keinoin (by this means).
In Finnish, a noun that is an attribute of another noun is normally in the genitive, e.g. Helsingin alue (Helsinki area), so there is no question of congruence. However, a title or a job description of a person may appear before a name, e.g. tohtori Lehto (doctor Lehto). Although it is somewhat debatable what the grammatical structure is in such expressions, the following principles are applied:
A proper name may have a common noun before it, characterizing the type of thing denoted, e.g. hotelli Adlon (hotel Adlon). When the common noun has no attributes, it is always uninflected, e.g. hotelli Adlonissa. When it has attributes, it is inflected, e.g. kaupungin parhaassa hotellissa Adlonissa (in Adlon, the best hotel in the city).
Finnish personal names are combinations of two or more nouns. In them, only the last part (surname) is inflected, e.g. Jukka Korpela : Jukka Korpelalle.
The congruence described above applies also to number, i.e. the use of plural versus singular. For example, we say iso talo (big house) : isossa talossa (in a/the big house) : isoissa taloissa (in big houses), where the last two words both have the plural suffix i before the case suffix ssa.
When the noun is plural in its form but singular in meaning (a plurale tantum word), congruence follows the grammatical form, e.g. kauniit kasvot (beautiful face).
A predicate and a predicative usually congrue with the subject in number, e.g. Tämä on hyvä auto (This is a good car) but Nämä ovat hyviä autoja (These are good cars), where all words are in plural form. This is described in more detail in chapter Singular and plural.
When the subject of a clause is a personal pronoun, the predicate is in the corresponding personal form, e.g. minä tiedän (I know), sinä tiedät (you know). This is an important phenomenon, but it is not congruence in the sense described above, since the grammatical form is not the same.
It is customary to say that the predicate congrues with the subject in number, even though the plural of a verb and the plural of a noun are different concepts. For example, we say ministeri tietää (the minister knows), but ministerit tietävät (the ministers know). There are some exceptions to such “congruence” in standard language, and in common spoken language, it is largely absent—we say ministerit tietää.