Answers the questions: What is it? Who is it? It is a Park. What is it? a park. It is Mark. Who is it? Mark. The park is green. What is green? the park. The man is walking. Who is walking? the man.
Usually used with the verb "to be"This is the case that simply names the entity in question and does not mark its dependence on any other word in the sentence. It is the case of the subject of the sentence.
In English, the role a noun plays in the sentence is indicated mainly by its position. When we hear Stan loves Barb we recognize Stan as the subject of the sentence (the source of the affection) and Barb as its object (the direct object of the sentence). In Barb loves Stan the different word order assigns different sentence roles to the two nouns.
In Polish, where "Stan" is Staś-, "loves" is kocha, and "Barb" is Baś-, it is CASE that communicates who loves whom. Case is indicated by the noun's case ending. NOMINATIVE case, which is indicated by a nominative-case ending (let us represent it as NOM), marks the subject of the sentence. ACCUSATIVE case, which is indicated by a accusative-case ending (let us represent it as ACC), marks the direct object of the sentence. So "Stan loves Barb" may be represented as Staś-NOM kocha Baś-ACC, and "Barb loves Stan" may be represented as Baś-NOM kocha Staś-ACC