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.
English pronouns come in three cases:
In
Latin,
nouns,
pronouns and
adjectives have the following cases:
Ancient
Greek 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.