Inflection of compounds

Normally only the last part of a compound word is inflected, e.g. kivitalo : kivitalossa, koeajaa : koeajoin.

If the first part of a compound verb is an adverbial, the compound is sometimes split and its order reversed in inflected finite forms. For example, the compound yhteensovittaa has participle forms like yhteensovitettu, but it is often split in finite forms. It is much more natural and more accepted to say Sovitin laitteen osat yhteen than Yhteensovitin laitteen osat.

If the first part of a compound is an adjective in the nominative, there are three possible inflection principles:

1. Only the second part is inflected, e.g. isoisä : isoisälle.
2. Both parts are inflected, e.g. omatunto : omantunnon.
3. Either just the second part or both parts are inflected, with no difference in meaning, e.g. harmaahaikara : harmaahaikara ∼ harmaitahaikaroita.

The inflection type is indicated in Finnish dictionaries, but in different ways. Suomen kielen perussanakirja and Kielitoimiston sanakirja describe words with inflection principle as having two possible inflection types. In the older Nykysuomen sanakirja, such words have a special inflection type (number 85). Often neither alternative has a clear majority. Moreover, dictionaries cannot cover all compounds, such as the large set of names of species of organisms. They normally follow inflection principle 3 especially if the first part is a name of a color in the nominative, e.g. mustatorvisieni : mustissatorvisienissä ∼ mustatorvisienissä.

In compounds with an adjective as the first part, it is increasingly common to leave that part uninflected. This is reflected in language standards. In newer dictionaries, e.g. harmaahaikara is described so that only the second part is inflected (principle 1) and isoveli as following principle 3, while earlier dictionaries specify principle 2, isoveli : isolleveljelle.