The suffix of the instructive is n. Thus, the singular form is the same as the genitive form, e.g. jalka : jalan (foot : by foot). The plural forms are different from the genitive, since the instructive plural is formed from the regular plural stem, e.g. jalo|i|n (cf. the inessive plural jalo|i|ssa), as opposite to the genitive, which is formed in different special ways, e.g. jalkojen (∼ jalkain).
There is never a possessive suffix in an instructive form. For example, we can say Tein sen omin käsin (I did it with my own hands), using the instructive of käsi : käden. If the adessive (‑lle case) is used instead, it takes a possessive suffix, e.g. Tein sen omilla käsilläni.
The instructive often indicates an instrument or tool, but it has varying phraseological use. The singular forms are almost exclusively adverb-like, e.g. jalan (by foot), and they are used for a small set of words only. However, the instructive is in productive use in II infinitive forms of verbs, e.g. kirjoitta|e|n (writing; by writing) and as adverb-like case forms of comparison forms of adjectives, e.g. nopeammin (faster), nopeimmin (fastest), which are plural instructive forms of nopeampi and nopein.
The plural forms are often used in a singular meaning. For example, Hän teki sen raskain sydämin (He did it with a heavy heart), the instructive raskain sydämin (of raskas sydän) is plural by form, singular in meaning.
Genuine singular forms of the instructive might still be seen in expressions like kahden käden käytettävä miekka (a sword used with two hands). However, such expressions are generally shortened as kahden käden miekka, where kahden käden can be interpreted as an attribute in the genitive.
The instructive is rare, except in specific phrases. Normally, an instrument is expressed using the adessive or other cases. We say matkustin junalla (I traveled by train); the instructive is be used in such expressions.
Instructive plural forms of cardinal numerals, such as yksin, kaksin, and kolmin, are used to mean “as a group of …”. The most common of these is yksin (alone). As a synonym for kaksin, the instructive singular kahden is also used.
It is often difficult to distinguish between expressions for an instrument of an action and the mode or conditions of an action. This is one reason why the instructive often denotes mode or style, as in He tekivät sen iloisin mielin (The did it in a happy mood; literally: … with merry minds). Here the adessive is possible, too: He tekivät sen iloisella mielellä. Generally, the adessive and the instructive are often used in such phrases rather interchangeably.
In some incongruent phrases like hyvillä mielin (in good mood), a noun in the instructive has an adjective attribute in the adessive.
The following phrase examples show how the meaning of the instructive varies from the indication of an instrument to describing mode, manner, or just context:
The common adverb hyvin (very) is originally the instructive of hyvä (good). This indicates that the use of the instructive for the mode of action is rather old, even though grammars have generally described it as the case of an instrument.
The word terveisin, often used in final greetings in letters, is by its form an instructive of terveinen (greeting), which is mostly used in plural. It can be accompanied with an adjective, e.g. parhain terveisin, rakkain terveisin, ystävällisin terveisin.
In advertizing language and similar styles, the instructive is often used in a manner similar to the use of the comitative. Due to rareness and similarity in form, these cases are often confused with each other. In slogans such as Joensuu virtaa uusin ilmein (Joensuu flows with new faces), the instructive is really used just as a counterpart to “with”. The same applies to phrases like kohtalokkain seurauksin (with fatal consequences).
Instruktiivin käyttö suomessa.