In Ithkuil there are only two lexical classes — formatives and adjuncts. Formative is the one functioning instead of a plethora of nouns, verbs, adjectives in other languages.
This is quite common in other languages as well for one verb to be able to function as both a noun, a verb and an adjective. For example, English ‘good’ can be both a noun and an adjective, and ‘stop’ is both a noun and a verb. Chinese ‘紅’ means ‘red’ as well as ‘redden’ and ‘redness’. The difficulty here is that you have to judge whether you see a verb or a noun depending purely by context.
Other languages give morphological cues by deriving relative words from the same stem. E.g. English ‘strength’, ‘strengthen’, ‘strong’ or Russian ‘чёрный’ (black), ‘чернеть’ (blacken), ‘чернота’ (blackness). Yet, the way natural languages employ derivation is unsystematic and haphazard.
Ithkuil addresses the issue the most elegant way by introducing the category of Function. Of course it would be too unsophisticated to have a category that directly maps into lexical classes common in natural languages. While two of the four Functions — Stative and Dynamic seem to correspond to nouns and verbs respectively, the distinction is much more subtle. If we ponder over the distinction between, say, a cat and being a cat, we won’t find any real difference. A cat is being a cat all the time, and the moment it stops being a cat, it is no longer a cat. Thus, the distinction is only in the perspective of the speaker — whether he sees being a cat as a dynamic action or as a relatively unchanging state.
Two remaining Functions also present different view on the matter. Manifestive Function puts the perspective of how the entity appears outwards, instead of focusing on the state or the action. In other words, it means ‘to be a cat (in name)’ or ‘manifest as a cat’. Incidentally, this is what we mean when we use the verb ‘be’ in English (as in ‘I am a cat.’) yet in Ithkuil you can equally use any other Function depending on what you think is most important in being a cat.
The last function is Descriptive, and while there is a temptation to map it to adjectives, it is better to think about in the Ithkuil way. It means ‘appear or manifest in the manner of’.
Function is shown by a vowel prefixed to the root consonant (when speaking about Ithkuil, it is convenient to refer to combination of vowels or diphtongs as ‘vowel’ and both single and stacked consonants as ‘consonant’). The vowel choice is different according to the Pattern and Stem of the formative and the given Function. All vowels are presented in the Table 8 of Chapter 5 of the grammar, the only table that I’d say is essential to memorise for convenience. At least the two rows of Stative and Dynamic are a must.
Finally, I’ll give all cat examples mentioned:
|Stative||rral||cat, a state of being a cat|
|Dynamic||irral||to be a cat, dynamic process of being a cat|
|Manifestive||uirral||to be a cat (in name), manifestation of a cat|
|Descriptive||oirral||to appear or manifest in a manner of a cat|