Finnish has often been characterized as an agglutinative language, in the typological classification of languages. This is misleading, however. A purely agglutinative language uses affixes (suffixes, infixes, or prefixes) that are “glued” to base words so that both the base word and the affix remain unchanged. In Finnish, the affix often varies, and it often causes changes in word stem.
Word inflection and derivation is partly agglutinative in Finnish; e.g. kouluni (my school) simply combines an unchanged base word koulu with an unchanged possessive suffix ni). Howeverm there is considerable variation both in stems and suffixes. The only stems with no variation are those that end with the short vowel o, u, ö, or y and that vowel is preceded by something else than a consonant k, p, or t participating in consonant gradiation. To be exact, there are also some word stems that invariably end with e (e.g. nalle : nalle|n), but in most stems, a final e varies with either i or ee (e.g., kivi : kive|n, aine : ainee|n).
Word stem means the part of a word that does not include suffixes (or other affixes). It is not always the same as the dictionary form, and it may vary between forms of the word. For example, the dictionary form rikas (rich) has a stem that differs from most case forms; the stem is rikkaa- in most singular forms and rikka- in most plural forms.
Most of the stem changes can be described in simple terms, though quite a few rules are needed to describe all the variation. A very common variation in the stem is a phenomenon called consonant gradation, e.g. so that the plural of kauppa (shop; trade) is kaupat, with the double pp turning to the single p when the plural suffix t is appended. In these examples, the stem has two forms, kauppa- and kaupa-; in plural forms, this word has additionally the stems kauppo- and kaupo-.
The most common variation in suffixes is vowel variation due to a principle called vowel harmony. For example, we say kaupassa (in a/the shop) but metsässä (in a/the forest), i.e. the case suffix has two forms ssa and ssä depending on what vowels appear in the base word.
In this book, we describe some of the stem variation by writing different forms separated by a colon “:”. This a common convention in Finnish grammars. For example, poika : poja- means that the word poika (boy; son) appears in inflected forms mostly as poja-, e.g. in the plural form pojat and in the genitive form pojan.
In addition to consonant gradation and vowel harmony, there is other variation in word stems in Finnish. A good example is verb inflection like juon : join (I drink : I drank), which seem to have the tense expressed by vowel changes in the verb stem, much like in the corresponding English verb. Similarly, the relationship poika : pojan (boy : boy’s), explained in the description of consonant gradation, is substantial word stem inflection.
Knowledge about the history of Finnish may help in understanding and remembering phenomena of the current language. For example, the verb stem juo- (to drink) is based on the earlier form with a long vowel, joo-, which is still in use in Estonian. In past tense forms, i was added to the stem, causing length reduction of the preceding vowel, and a diphthong was born: joi-. Therefore, the past tense forms did not participate in the change that produced juo-.
By convention, the basic form of a word is
In Finnish, as in many other languages, the “basic form” (nominative singular) of a noun is often more or less exceptional and deviates from most other forms. For example, words that end with s in the “basic form” have varying types of stems in most other forms, such as tulos : tuloksen (result), with the stem tulokse-, and vapaus : vapauden (freedom), with the stem vapaude-. Language learning might be somewhat easier if dictionaries had the inflectional stem rather than the nominative singular as the key form.
One reason why the basic form is often exceptional is that some phonetic changes have taken place only at the end of a word. For example, at some point of language history, a word-final e was changed to i. Words like kivi originally ended with e in the basic form, too, and had regular inflection: kive : kiven : kivessä etc.