Adverbs are words that are used to complement a word or an entire sentence. A typical example is Tein sen eilen (I did it yesterday), which consists of a predicate, an object, and an adverb, where eilen can be interpreted as complementing the entire sentence or just its predicate tein.
The concept of adverb are best defined negatively: an adverb is a word that is not a noun-like word or a verb and that is used to complement an expression. For example, the sentence Tein sen tiistaina (I did it on Tuesday) contains the complementing word tiistaina, but it is a regular inflection form (essive) of the noun tiistai.
Adverbs are often characterized as having no inflection, but this is not quite true. First, the difference between an adverb and an inflected form is often a matter of definition. For example, kerran (once) is classified an adverb in dictionaries, but it could alternatively be classified according to its origin: as the instructive singular of the noun kerta. Second, in Finnish there are sets of related adverbs that have case suffixes, e.g. sisällä, sisältä, sisälle refer to being inside, coming from inside, and getting inside something. They could be described as inflected forms of the noun sisä with limited inflection. Such sets are discussed in section Locational adverbs.