Infinitives are classified as forms of verbs, but they behave much like nouns. In Finnish, they may have case suffixes and possessive suffixes. Their use in a sentence often corresponds to the use of nouns. Compare the sentences Haluan leipää (I want bread) and Haluan syödä (I want to eat). In the latter, the infinitive syödä is the object. Finnish infinitives lack tenses, moods, and number (i.e. singular vs. plural).
On the other hand, infinitives are formed using systematic inflection. They also have properties of verbs: they can have objects and adverbials. In Haluan syödä aterian täällä (I want to eat a meal here), aterian is an object of the infinitive syödä, and täällä is an adverbial of syödä, not of the predicate verb haluan.
In English, the infinitive has no suffix but it is usually preceded by “to”. In Finnish, there is no counterpart to the word “to” in infinitives. Instead, infinitives have suffixes that indicate a specific type of infinitive.
In addition to the basic infinitive form such as sanoa “to say”, Finnish has several other infinitives. The types of infinitives have traditionally been numbered as follows:
As in English, the basic infinitive is used in conjunction with verbs expressing wishes, possibilities, capabilities, etc., e.g. Tahdon lähteä pois (I want to go away), Osaan ohjelmoida (I know how to program). However, as described in the next section, some verbs require the use of other forms than the I infinitive.
The other infinitives are used in various expressions that can be classified as clause equivalents. Sometimes they correspond to “-ing” forms in English, e.g. hän käveli katsellen lintuja (he walked watching birds), where katsellen is a II infinitive form. Although this form is in the instructive case, it does not usually denote an instrument but rather refers to something happening simultaneously with the action expressed by the predicate.
In colloquial language, infinitives other than the basic one are rare, mostly occurring in sayings only, such as sanomatta mitään “without saying anything”. However, the inner locational cases of the III infinitive are used in speech, too, e.g. mä käyn ostamas maitoo (standard Finnish: käyn ostamassa maitoa “I’ll go to buy milk”) In nonfiction prose, the II and III infinitives are common, though this depends on writing style.
All modern dictionaries use the I infinitive as the basic form (lookup form) for a verb. This is somewhat unfortunate for several reasons. The infinitive is not the most common form, and it often has specialties in its form. It often has a somewhat peculiar formation; e.g., juosta (to run) has the stem juos-, but most forms of this verb have the stem juokse-. A few verbs do not even have the I infinitive in use (e.g. erkanee “is separated”; its infinitive erata is just a theoretical construction).
To deal with some of the problems, verbs are sometimes referred to by their typical stem, such as juokse-, or the indicative present tense 3rd person singular, like juoksee. The reason is that the formation of verb forms is easier to understand with the stem as the starting point, instead of the infinitive. Some old dictionaries use the indicative first person singular, such as tulen; the most adequate form would be the indicative third person singular, such as juoksee, since all verbs have this form in actual use. This is actually the form used in Kielitoimiston sanakirja for those few verbs that lack the I infinitive, such as ei, erkanee, kumajaa, kaikaa.
Knowing the dictionary form is not sufficient for constructing all the forms of a verb. However, this mostly depends on consonant gradation and on some irregulaties in the inflection of some verbs. For example, there are two different verbs with the same I infinitive form tavata; in indicative 3rd person singular, they are tapaa (meets) and tavaa (spells).