The suffix of the illative has several forms, e.g. kala : kala|an : kalo|i|hin (fish), puu : puu|hun : pu|i|hin (tree), and vene : venee|seen : vene|i|siin (fish). The suffixes can be described as *n, h*n, seen, and siin, where the asterisk * denotes a vowel that is identical with the preceding vowel.
When the suffix is *n, it phonetically means that the last vowel before it is lengthened and then an n is appended.
The illative suffixes used for a word are determined by the inflection stem of the word. However, in some words, either h*n or seen can be used in singular and either hin on siin in plural.
In foreign words that end with a long vowel or diphthong, the suffix h*n may have the vowel either according to the pronunciation or according to the spelling of the base word. For example, the illative of fondue [fondyy] can be either fonduehyn or fonduehen, and the illative of jockey This awkward situation can often be avoided by using alternative word forms adapted to Finnish writing, such as fondyy : fondyyhyn.
In foreign words that end with a short vowel written in a manner different from Finnish spelling, the vowel used in the *n ending is usually the last written vowel. For example, the illative of smoothie [smuuði] is smoothieen [smuuðiin], though this form can also be written as if the base form were pronounced with ie at the end, i.e. smoothiehen.
The original illative suffix has probably been zen, but the z sound (now absent in Finnish) has partly developed to s, partly to h, which has often vanished. Moreover, the e has widely been assimilated to the preceding vowel.
The rules for selecting the singular suffix are:
The inflection stem cannot end with a consonant. Words ending with a consonant in the basic form have a vowel stem used in inflection, e.g. lyhyt : lyhye|n : lyhye|en, or they are foreign words with a binding vowel i, hence take the in suffix, e.g. Oxford : Oxfordi|n : Oxfordi|in.
In plural, the illative suffix is always preceded by an i acting as the plural suffix. Therefore the illative suffix is in, hin, or siin as follows:
There is a small set of words that end with ut or yt in the basic form without being participles of verbs (such as sanonut): ainut, airut, ehyt, immyt, kevyt, kytkyt, kätkyt, lyhyt, ohut, olut, neitsyt, tiehyt. They have an inflection stem with an e instead of the t. Since ue and ye are not diphthongs in this position, the illative suffix is *n, e.g. kevyt : kevye|n : kevye|en. In plural, both siin and hin are possible, the former being more common; e.g. kevy|i|siin ∼ kevy|i|hin.
However, this inflection type is very often confused with the type of words ending with Uex, with inflection stem ending with Uee, e.g. puolue : puoluee|n : puoluee|seen. This causes illatives like kevyeeseen. Though still substandard, they are described in the modern descriptive grammar Iso suomen kielioppi.
When the stem ends with a short vowel, the suffix is *n, and here * stands for the final vowel letter, even if its pronunciation differs from Finnish rules. Examples: Alaska : Alaskaan, Sydney [sidni] : Sydneyyn [sidniin], Loire [luaar] : Loireen [luaareen] ∼ [luaariin]. As the last example shows, a mute final e is doubled in writing, but the pronunciation may be [ee] or [ii].
When the stem ends with a diphthong, the suffix is h*n. Words of this type are rare, e.g. Weihai : Weihaihin. Words like Marseille and Versailles have traditionally been interpreted as ending with a diphthong, e.g. Marseille [marsei] : Marseillehin [marseihin], but now they may be treated as ending with [j], e.g. Marseille [marsej] : Marseilleen [marsejiin]. On the other hand, the latter spelling may be used even when the pronunciation is traditional.
When the stem ends with a long vowel, the suffix can be h*n or seen. The rules for this are:
When the suffix is h*n, the vowel in it is written according to Finnish orthography. Thus, although the vowel sound is the same as the final vowel sound of the base word, the letter need not match the final vowel letter. Example: château [šatoo] : châteauhon [šatoohon]. However, sometimes the final letter is used, e.g. spray [spreihin] : sprayhin ∼ sprayhyn [spreihin], and this is implicitly accepted: it is described as an allowed variant in Kielitoimiston sanakirja.
If the spelling of the basic form ends with a consonant but the pronunciation in a vowel, an apostrophe is used between it and the illative suffix, like for other suffixes. Example: gourmet [gurmee] : gourmet’hen [gurmeehen].
Foreign words that end with a long vowel have typically the final vowel stressed in the original language. This may explain why they take the h*n suffix: the inflection follows the pattern of one-syllable words with a long, stressed vowel, as in maa : maahan.
Loanwords that are more adapted to the Finnish language tend to be inflected according to the pattern of Finnish words with an unstressed long vowel, e.g. takuu : takuu|seen.
Such processes of adaptation are reflected in words like gurmee, which is the word gourmet written as an adapted loanword. For it, alternative illative forms exist: gurmeehen ∼ gurmeeseen.
Illatives of proper names have some specialties:
In words that have consonant gradation, the grade is strong in illative forms, even when basic gradation rules would imply weak grade, e.g. kunta : kunta|an : kunt|i|in, as opposite to weak grade in many other case forms: kunna|ssa, kunn|i|ssa etc. The reason is that these forms originally had (and still have in some dialects) an h, e.g. kuntahan : kuntihin.
However, the weak grade is also possible in illative plural forms of some words:
The illative often corresponds to the English proposition “into” or “to” and usually refers to being moved into something, in a concrete or abstract sense. Examples: taloon (into a/the house), tammikuuhun asti (until January), joutua vaikeuksiin (to get into trouble).
The illative is also used in contexts where it means entering a state of being in close contact with or attached to something, as in panna käsineet käteen (to put gloves on; literally: to put gloves into hand). Section Locational cases covers such uses of the illative.
The illative also has many grammatical uses, as in Rakastuin sinuun (I fell in love with you) and Uskon Jumalaan (I believe in God). The verb uskoa, when used to express faith, does not normally have an object; instead, an adverbial in the illative is used. Similarly we say e.g. Luotan sinuun (I trust on you). The verb kuulua (to belong) takes the illative when it expresses that something is part of something else, e.g. Hintaan kuuluu aamiainen (The price includes breakfast); when it means belonging to someone, it takes the allative (-lle case), e.g. Tämä kuuluu Annalle (This belongs to Anna). Other grammatical uses of the illative include the following: