The instructive plural of comparative and superlative forms are almost exclusively used as adverbs. This means that these forms effectively create comparison forms of adverbs derived with the sti suffix. Example:
There are also adverbs like these but with a base word that does not exist in modern language. For example, the base word of mieluummin (rather, preferably) and mieluimmin (best of all, most preferably) would be mieluu, but it does not exist in current language.
Similarly to the extended use of the superlative, as in mitä nopein (very, very rapid), the superlative adverb may be prefixed with mitä, e.g. mitä nopeimmin (very very rapidly, really fast).
The equivalents of the English adverbs “well”, “better”, and “best” are hyvin, paremmin, parhaimmin ∼ parhaiten. The word hyvin is originally the instructive plural of hyvä (good). The adverb hyvästi also exists, but in modern language, it relates to saying goodbye (Jää hyvästi or just Hyvästi).
There are also some other instructive plural forms of basic adjectives that are used as adverbs, such as harvoin (rarely), kauan (a long time), kovin (very; cf. kova “hard”), oikein (right, correctly), pahoin (badly), samoin (similarly), tosin (surely but…; cf. tosi “true”), usein (often), väärin (wrong). The adverb aikaisin (early) belongs to this type, since it is the instructive plural of the adjective aikainen (early), not its superlative, which has the same form.
The comparison forms of instructive forms used as adverbs are derived from the base adjective. E.g. for pahoin, they are derived from the comparison forms of the adjective paha: pahemmin, pahimmin. For aikaisin, the comparative is aikaisemmin, but the shorter aiemmin (derived more directly from the base word aika) is also used.
Some adverbs are originally instructive plural forms of nouns, e.g. osin (partly), tuskin (hardly; cf. tuska “pain”), and vaivoin (with difficulty). They do not have comparison forms.
The adverb vähän (a little) can be interpreted as instructive singular of the adjective vähä (small). The adjective is otherwise rare in modern language, except in proper names like Vähä-Aasia (Asia Minor), but it but has many widely used derivations. The derivations include the comparison forms vähempi (lesser) and vähin (least) and the corresponding adverbs vähemmän and vähiten ∼ vähimmin.
The adverb paljon (a lot) is based on paljo, which is used both as an adjective and as a noun to express a large amount. It has no comparison forms. Instead, the adjectives enempi and enin and the corresponding adverbs enemmän (more) and eniten ∼ enimmin (most) are used. They are based on the stem enä, which is not used as such in modern languages except in some proper names lik Enäjärvi and as the partitive form enää, which is used as an adverb in negative contexts, e.g. En muista sitä enää (I do not remember it any more). A common substandard form, used in some dialects, is enään.
The adverb kauan (a long time) is based on the same stem kauka- : kaua- as the locational adverbs kaukana, kaukaa, and kauas. The comparative and superlative adverbs are based on that stem: kauemmin (a longer time), kauimmin (the longest time).
In a negative clause, the partitive paljoa is often used, though paljon is permitted, too: Se ei maksanut paljoa ∼ paljon. This corresponds to general rules for the case of an object. Similarly, kauaa is often used instead of kauan in negative clauses, e.g. En ollut siellä kauaa ∼ kauan (I did not stay there long). However, such use was not accepted by language authorities before 2015.
The adverb myöhään (late) is the illative of myöhä (late time), on which the comparison forms are based: myöhemmin (later) and the rare myöhimmin (most late).
The adverb ennen (before, earlier), also used as an adposition, expresses comparison as such. The corresponding comparative form ennemmin the same meaning, but it is also used to express preference, “rather, sooner”.
A genitive form of an adjective can be used in adverb-like manner to qualify another adjective or an adverb, e.g. tavattoman halpa (unusually inexpensive). In English, an adverb ending with “-ly” is typically used instead.
Along with parhaimmin, the adverb parhaiten is also used, and actually much more common. Similar alternatives, formed by adding the iten suffix to an adjective stem, exist for many adverbs, though some of them are colloquial in style. The following are standard and possibly more common than the -immin words (shown here in parentheses): eniten (enimmin), mieluiten (mieluimmin), pahiten (pahimmin), parhaiten (parhaimmin), vähiten (vähimmin). Opinions disagree on the suitability of words like helpoiten (instead of helpoimmin), huonoiten (instead of huonoimmin), and nopeiten (instead of nopeimmin). They are not listed in dictionaries of standard Finnish.
The -iten derivations are usually based on a stem that is either two-syllabic, like paha, or ends with -ea, e.g. nopea.
Adverbs with the iten ending also appear so that the ten suffix is appended to the normal plural inflection stem of a superlative form, e.g. nopeimmiten, based on nopein : nopeimmissa. These forms do not substantially differ from the use of instructive plural forms of superlatives, such as nopeimmin. However, they are often more easily understood as adverbs rather than as forms of adjectives.
On the other hand, there are specialized meanings: pikimmiten, hetimmiten, and kiireimmiten all mean “as soon as possible” (with varying tone), whereas vanhemmiten means “as one gets older” or “later in life”. The word useimmiten means “usually, as a rule”, so it is rather close in meaning to useimmin (most often).