In addition to usage described in the preceding sections, there are some other uses of case forms of noun-like words as adverbs. There is no strict line between such use and normal use of case forms. Some case forms have been “petrified” so that they have become rather purely adverbs, whereas some other might still be used as normal inflected form.
The classification of words as case forms or as adverbs has little if any impact on practical use of the language or on the rules of the language. The old dictionary Nykysuomen sanakirja categorizes words by their part of speech, but newer dictionaries like Kielitoimiston sanakirja do not; yet they indirectly classify words as adverbs with the note Sana on taipumaton tai vaillinaisesti taipuva (The word has no inflection or defective inflection).
Adessive (-llA case) forms of nouns are often used in an adverb-like manner to denote the style or kind of an action, e.g. kiireellä (hastily), rakkaudella (with love), kunnioituksella (respectfully), lämmöllä (warmly). Language guides have regarded such usage as substandard, suggesting the use of derived adverbs or verb forms instead, such as kiireellisesti, rakastavasti, kunnioittaen, lämpimästi. However, the adessive forms are widely used, and they are mostly accepted in standard language now.
The use of abessive (-ttA case) forms of nouns is mostly limited to adverb-like words. For example, the word syyttä (without reason) is by its form the abessive singular of syy (reason, cause), but it is often understood as an adverb. For example, syyttä can hardly take an attribute in spoken language, but in literary style, we could write hyvättä syyttä (without a good reason), though this sounds old-fashioned.
There are words that describe states of mind or things, such as eksyksissä, eksyksistä, and eksyksiin, which refer to being astray, getting away from that state, and getting into it, respectively. By their form, they are the inner locational cases, i.e. inessive, elative, and illative (all plural) of eksys : eksykse-, but such a word does not exist in any other form than those three. Other sets of words of this type exist in outer locational cases only, e.g. valveilla, valveilta, and valveille, related to being awake. In such groups, the elative or the ablative, e.g. eksyksistä and valveilta, are rarely used.
Such forms are described in dictionaries as adverbs, either as groups or each form as an entry of its own.
Similar groups of words appear also with a normal noun as their stem. For example, hengissä and henkiin (alive) could be classified as adverbs by their use, but they are the inessive and illative plural of henki (spirit; mind; life). For example, jäädä henkiin (to stay alive) literally means “to remain into lives”. The word innoissaan and intoihinsa are forms of the noun into (enthusiasm), but their use, in plural and with possessive suffixes, make them adverb-like. However, dictionaries usually describe such usage under the entry for the noun, rather than as separate words.