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There is a class of invariable words, able to modify nouns and verbs, adding a specific meaning, wether semantical or deictic. They can be independent words (Adverbs); prefixes of verbal stems (Preverbs), originally independent but usually united with it; and also a nexus between a noun and a verb (Appositions), expressing a non-grammatical relationship, normally put behind, but sometimes coming before the word.
NOTE. As usual, in the oldest IE the three categories were probably only different uses of the same word class, but they were eventually assigned to one function and meaning. In fact, Adverbs are generally distinguished from the other two categories, so that they change due to innovation, while Preverbs and Appositions remain the same and normally freeze in their old positions.
Adverbs come usually from old particles which have obtained a specific deictic meaning. Traditionally, Adverbs are deemed to be the result of old oblique cases which have frozen in modern languages, loosing inflection. However, this is not the origin of the oldest types (a lot of them being found in modern Europaio), which are not related to older nouns, but to old (possibly general noun-verb) roots.
- Quiles Casas, Carlos, Europaio: A Brief Grammar of the European Language, Vol. 1, Dnghu, 2006, ISBN 84-689-7727-6