Essive case

In grammar, the essive case, or similaris case, (abbreviated ) is a grammatical case. The essive case on a noun can express it as a definite period of time during which something happens or during which a continuous action was completed. It can also denote a form as a temporary location, state of being, or character in which the subject was at a given time. The latter meaning is often referred to as the equivalent of the English phrase "as a __".
Finnish.
In Finnish, it is marked by adding "-na/-nä" (depending on the vowel harmony) to the stem of the noun.
Use of the essive case for specifying times, days and dates when something happens is also apparent in Finnish.
In Finnish, the essive case is technically categorized as an old locative case, a case that, in some way, indicates spatial location. However, in the present language, the case has lost the majority of its spatial meaning. The case instead typically denotes a state that is temporary or inclined to change.
Some fixed expressions retain the essive in its ancient locative meaning, however: "at home" is "kotona". 
If the inessive were used, "kodissani", it would distinguish the activity from reading the papers, such as in the garage or in the garden (of the home).
When marking something that cannot literally change states, the essive case can implicate the presence of alternative states, even two individual, differing "worlds". That can be seen in the following example:
The example above illustrates the process by which marking of the essive case can be seen as creating two differing "worlds": one real and one illusionary. The "temporary" component of the meaning encoded by marking of the essive case on the Finnish word for "genuine" (aito) makes a distinction between the perceived state of the subject, as genuine at the time of purchase, and the actual state of subject, as not genuine as it is now perceived or at the time of the moment of speech.
Estonian.
In Estonian, it is marked by adding "-na" to the genitive stem. Marking of the case in Estonian denotes the capacity in which the subject acts. The essive case is used for indicating "states of being" but not of "becoming", which is instead marked by the translative case, the elative case, or the nominative case.