The essive suffix is always na or nä, e.g. talo : talo|na : talo|i|na, ihminen : ihmise|nä : ihmis|i|nä. As explained in section Vowel stem vs. consonant stem, many words have an alternative (often outdated or poetic) essive form based on a consonant stem, e.g. vuosi : vuote|na ∼ vuon|na.
The essive usually indicates some sort of role or state, which may be permanent or temporary. For example, “he worked as a teacher” is hän työskenteli opettajana, with the last word opettaja (teacher) in the essive. Such an essive form refers to the subject of a clause, or sometimes to the “logical subject”, as in Minulla on tapana työskennellä myöhään (I use to work late; literally “at me is as habit to work late”). It may also refer to the object, usually with no real risk of misunderstanding, as in Lähetin sen tavallisena kirjeenä (I sent it as an ordinary letter).
The essive can also be used as a predicative, as in Hän on opettajana Turussa (He/she works as a teacher in Turku). It indicates temporary role or state, as opposite to sentences with the nominative such as Hän on opettaja (He/she is a teacher).
The essive is common in expressions like nuorena (as young) and vanhana (as old). Such expressions are usually translated into English using subordinate clauses, e.g. nuorempana pelasin paljon šakkia as “when I was younger, I played chess a lot” (rather than “as younger…”).
The essive is also used in expressions like Tämä sopimus on tehty kahtena kappaleena (This contract has been made in two copies) and when expressing a unit, e.g. Paljonko maili on metreinä? (How much is a mile in meters?) and Hinnat on ilmoitettu euroina (The prices are given in euros). However, for units, the inessive is used more often, e.g. metreissä, euroissa, and this is now accepted by language authorities.
An expression in the essive sometimes expresses a cause or a reason, much like an “as …” expression in English. Example: Punatukkaisena minun on vältettävä polttamasta itseäni auringossa (As redheaded I need to avoid getting sunburnt). Here punatukkaisena does not mean “when I am redheaded” bu rather “because I am redheaded”.
When the essive is used with the verb olla (to be), in a construct called predicative adverbial, it says that the state is temporary. For example, Olen sairas (I am sick) may indicate permanent or temporary property, whereas Olen sairaana means that I am sick at the moment, suggesting that the condition is temporary.
The essive is used with verb pitää in the meaning “to regard as” as the required case, as in pidin häntä luotettavana (I regarded him as reliable). Such usage might be seen as indicating a state, in a way.
The essive was originally a locative case, later mostly replaced by the inessive and the adessive, as explained in section Locational cases. The old meaning is still preserved in some adverbs, like kotona (at home) and ulkona (outside) and in comparatives of nouns, e.g. rannempana (closer to the shore).
Moreover, the essive has temporal usage in some contexts, like maanantaina (on Monday); see Expressions of time.