The Essive case reference article from the English Wikipedia on 24-Apr-2004
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Essive case

Time you got around to sponsoring a child
Cases
Abessive case
Ablative case
Absolutive case
Accusative case
Adessive case
Allative case
Comitative case
Dative case
Dedative case
Elative case
Ergative case
Essive case
Genitive case
Illative case
Inessive case
Instrumental case
Locative case
Nominative case
Oblique case
Partitive case
Possessive case
Postpositional case
Prepositional case
Prolative case
Terminative case
Translative case
Vocative case
Declension
List of cases

The essive case carries the meaning of a temporary state of being, often equivalent to the English "as a ...".

In the Finnish language, this case is marked by adding "na" to the genitive stem of the noun, but with strong consonant gradation. Example: "lapsi" -> "child", "lapsena" -> "as a child", "when (I) was a child".

In Finnish, it is also used for specifying days and dates when something happens. For example: "maanantaina" -> "on monday", "kuudentena joulukuuta" -> "on the 6th of December".