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| Cardinal : kahdeksan|
Ordinal : kahdeksas
Adverbial : kahdeksasti
Multiplier : kahdeksankertainen
Colloquial counting form : kasi
Colloquial ordinal : kakki
|Finnish Wikipedia article on kahdeksan|
From Proto-Finnic *kakteksa, i.e *kakt-e-ksa, consisting of an earlier form of kaksi + a form of the negation verb ei + ksa, which has the same origin as the endings in the reflexive forms found in dialects, c.f. Kalevala forms such as istuikse = istuutuu (“he/she sits”), so that the combination means "two are not there", and "of ten" was implied or originally said. Compare yhdeksän (“nine”), which developed the same way.
The original nominative kahdeksa was replaced by the genitive due to kymmen ending in -n (which is also the genitive ending), but is attested in older texts, particularly in compounds.
Declension of kahdeksan (type koira)
Declension is the same as type koira (number 10), except of the nominative singular form (*), where an -n is added at the end.