It is noticeable that generally when this case is used, either a noun with an adjective is found or two nouns both in the instructive. Thus one can say kissa repi ovea kynsin hampain, whereas if only one word were used it would be better to use kynsillä or hampailla.
The instructive rarely takes pronominal suffixes. Kal. ix. 507, 509, En liiku omin lihoini ... En väiky omin väkini.
II. The instructive of adjectives is frequently used adverbially to express manner.
(1) The instructive singular of an adjective is used adverbially to qualify another adjective or adverb in the positive. Tavattoman kaunis, exceedingly beautiful; kauhean ruma, horribly ugly; hyvin hyvä, very good; tämä yö oli hirveän pitkä, the night was terribly long.
(2) The instrumental plural of adjectives in the comparative or superlative is used to define verbs. Puhu nopeammin, speak quicker; aurinko paistaa kirkkaammin, the sun shines brighter.
III. The instructive plural is also used in a temporal sense, and denotes an indefinite or recurring period of time. Thus linnut laulavat illoin, aamuin, the birds sing in the morning, in the evening (whereas if a definite morning or evening is meant the essive must be used). In this sense the termination sin is often used, which is really the instructive plural of an adjective with a nominative nen formed from the noun. Thus me alamme työn aamuisin kello kahdeksasta, we begin to work at eight in the morning. Iltaisin rupeamme aikaisemmin maata, we go to led earlier in the evening. Hän tekee talvisin kauppamatkoja, he makes business journeys in the winter. Onko teillä tapa valvoa öisin? are you in the habit of not sleeping at night?
This same ending is added to substantives in a sense somewhat analogous to the prolative. Minä menen rautateisin, I go by rail. Sotaväki matkusti osaksi maisin osaksi merisin, the troops travelled partly by land, partly by sea.
Prolative.
This case is rarely used, and in fact has no sense when formed from any but substantives expressing some kind of local position. It expresses motion by or over anything, and is chiefly employed in adverbial expressions, and generally in the plural. Tulla meritse tai maitse, to come by sea or land. Järven ranatse, along the shore of the lake. Niemen nenitse, round the promontory. Aalloitse, by sea. Minä kylvän kyyhättelen Luojan sormien lomitse, I sow by the finger of God (lit. through the interval of the fingers). Kal. ii. 297 (Mehiläinen lenti) Otavaisten olkapäitse, Seitsentähty en selitse, (the bee flew) over the shoulder of the great bear, over the back of the seven stars.