Nouns, pronouns and adjectives of inflected languages decline into different cases, with different grammatical roles, in the same way that verbs conjugate into different tenses and voices.
s come in three cases:
s and adjective
s have the following cases:
has the following cases:
As with English, Latin and Greek nouns, pronouns and adjectives can generally be singular or plural in the different cases. (Greek has a dual as well, but let's not go there).
In addition to the uses listed above, most of the cases have idiomatic usages. A good grammar will detail these.
In bookbinding, a case is one kind of cover for a hardback book. See casing in for how to make one.