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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. The initial component is kaksi (“two”), but the nature of the latter component (shared with yhdeksän (“nine”)) remains disputed. The most common explanation is that it consists of two parts: *e-, the negative verb (ei), and *-ksa, a reflexive verb ending (compare Kalevala forms such as istuikse = istuutuu (“he/she sits (down)”)). The combination would then mean "two are not there", and "of ten" was implied or originally said.
The original nominative kahdeksa was replaced by the genitive form due to seitsen and kymmen ending in -n (which is also the genitive ending), but is attested in older texts, particularly in compounds. Alternatively, if the aforementioned explanation is true, the final *-n may be an old dual verb ending, in which case it may be original in kahdeksan and analogous in yhdeksän.
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.