A derived word consists of the stem of a base word and a suffix. For example, the word sanasto (vocabulary) consists of the stem of the base word sana (word) and the suffix sto, which has a collective meaning. Here the stem of the base word is the same as its basic form in dictionaries, the nominative singular.
The base word may itself be a derived word. For example, from sanasto we can derive sanastollinen (lexical).
The Suffix glossary in this book lists the most common suffixes in Finnish, including derivational suffixes. As it shows, the same suffix may appear both as a derivational suffix and as an inflection suffix, though this is relatively rare in Finnish.
Suomen kielen sidonnaiset morfeemit (contains an exhaustive table of suffixes in standard Finnish).
In most words derived from nouns and adjectives, the stem of the base word is the form used in the genitive singular. For example, adding the general suffix llinen to the word asenne : asenteen (attitude; prejudice) yields asenteellinen (prejudiced; related to attitudes), i.e. the base word stem is here asentee-.
For words derived from verbs, the stem of the base word is mostly the same as in the 1st person singular, indicative present tense. For example, from juosta : juoksen (to run) we can derive the word juoksettaa (to make something/someone run), where the base word stem is juokse and the suffix is ttaa.
The stem of the base word may vary due to consonant gradation and other changes, as described in section Other variation in word stem.
A final short vowel is normally omitted before a suffix that starts with a vowel. For example, when the suffix u is appended to the verb stem juokse- to form a name of action, the e is lost: juoksu (run). This also happens when the final vowel is preceded by another vowel, e.g. urhea (brave) → urhe|illa (to go in for sports) , here causing the stem vowel e and the suffix vowel i to form a diphthong.
A final long vowel is shortened before a vowel, not lost. Example: vene : venee|n (boat) → vane|illä (to boat; to go boating).
Sometimes a derived word concides with an inflected form of the base word. The verb infinitives mentioned above, urheilla and veneillä, are written the same way as the adessive plural forms of urhea and vene. In spoken Finnish, the words differ, since the infinitives end with boundary gemination.
Some words have been created by altering another word in different, often irregular ways and possibly adding a suffix. Such creations are very common in slang and jargon. They often produce a more convenient word that can be used instead of a long and complicated word. However, probably a stronger reason for using such words is that they sound informal.
For example, from valkoviini (white wine), itself a normal compond word, the colloquial valkkari has been produced. In it, either ri or ari can be regarded as a suffix, especially since such suffixes are common in Finnish words. The stem valkka or valkk is irregular, however; we can say that it is motivated by the word valkoviini, or its start, rather than just its modified form. Such colloqualisms are often used in different environments in different meanings. For example, among bird hobbyists, valkkari may mean valkoposkihanhi, a bird species (barnacle goose; the Finnish name means “white-cheeked goose”).
It is a matter of definition whether such alterations are called derived words.
Omitting the end or the start of a word can be regarded as the simplest form of alteration, a shortened word. It is common in spoken language, but seldom accepted in formal language. For example, ale (genitive: alen) is a truncation of alennusmyynti (sale) and very common in advertisements, but normally not used otherwise in written language. The word info for informaatio is a common colloquialism, whereas ope for opettaja (teacher) is school language.
Some truncations appear only as the first component of a compound word, often imitating similar use in other languages, e.g. digi- (= digitaali-). They can be classified as compositive forms.
Some truncations are abbreviations treated as words. For example, esim. is common abbreviation for esimerkiksi (for example) and is read as that full word in formal language, but informally it can be read as just esim and even written that way, without a period.
A special case is suht. (= suhteellisen, relatively), which is rather rare as an abbreviation in writing, but common as a truncation spoken language. It is often combined with koht. (= kohtuullisen or kohtalaisen, meaning “reasonably” or “moderately”), written varyingly suht. koht. or suht.koht. or suht koht or suhtkoht, meaning “fairly, rather”.
The word “acronym” has several meanings, but here we use it denote a word formed from initial letters of words, such as hetu from henkilötunnus (a compound of henkilö and tunnus). Such words are common in modern Finnish, but usually colloquial. They are used like normal words, but single stops k, p, t do no participate in consonant gradation; e.g. the genitive of hetu is hetun, not hedun.
In a more restricted sense, an “acronym” is formed from first letters of words. Such acronyms are rare in Finnish. They are often loanwords so that the word is not an acronym in Finnish, e.g. rem (rapid eye movement), vip (∼ VIP), Nato (∼ NATO).
New acronyms are often created because a new administrative or other term is an inconveniently long expression like aluehallintovirasto and an abbreviation is like avi (∼ AVI) is defined for it. If the abbreviation is a pronouncible word, it easily becomes an acronym: it is no longer spelled out, except in the most formal contexts.
Many acronyms are used in a restricted way only, at least in formal language: as the first component of a compound word. Such words are often base on a pair of words. They include luki- (from luku ja kirjoitus) in lukihäiriö (dyslexia), hevi- (from hedelmä ja vihannes) in heviosasto (fruit and vegetable department), and sote- (from sosiaali- ja terveydenhuolto) in soteuudistus (social welfare and health care reform). In informal language, however, such words may appear as such, e.g. sote standing for soteuudistus.
People learn Finnish derivation suffixes as they learn words containing them, rather than by learning the suffixes separately. This applies to learning Finnish as first language (native language) as well as other forms of learning. However, it is possible to assist such natural learning processes by a more systematic study of suffixes.
It is easy to notice similarities in meanings of kirjasto, astiasto, laivasto etc. when you learn such words, and this may make you expect that sto always has a similar meaning as a collective suffix. One day, you might even dare to coin a new word using it, for example kalasto, from kala (fish). That’s normal in Finnish. Some suffixes are so productive that forming a derivation is not a big thing, maybe even just comparable to using two or more words together (an open compound, or a phrase like “set of fish”) in English. But there are some pitfalls. For example, kalasto is an existing word, but it has a specific meaning; it does not mean any set or collective of fish but the totality of all fish in some water area.
Among the problems of spelling related to derivational suffixes, the one that disturbs native speakers most is the issue of -ottaa vs. -oittaa verbs. The official rules are complicated. The base rule is that the suffix is -oittaa if the stem of the base word ends with a. For example, we have haavoittaa (to wound) from haava (wound), but tiedottaa (to inform) from tieto (information). However, there are many exceptions, and it is often not clear what the base word is.
In spoken language, most -oittaa verbs appear without the i, e.g. haavottaa. This makes the issue difficult to native speakers when they try to write correctly or speak formally.
However, in both written and spoken language, the -oittaa form is always used when the base word has three syllables. Example: vahingoittaa (to harm), from vahinko (damage).