Case (grammar)

In grammar, case changes what a noun, adjective or pronoun does in a sentence. It is a set of forms which depend on the syntax (how the words go together). Case is an example of inflection, which is often an affix, a part of a word that is added to other words, that signals a grammatical relationship. Long ago, Old English used several cases, but Modern English does not use cases except for in pronouns. 
History of case.
In Latin, nouns pack several ideas into one word:
In many languages like Latin, German, Russian, Korean and Japanese, a noun's case changes the end of a word depending on the noun's role in the sentence. Nouns change their endings to show that they are doing something, that something is done to them, that they just happen to be there during the action, or that they own something.
Therefore, word order in those languages is less important than in English, which often has word order change a sentence's meaning.
Modern English.
In English, case is not used much. Instead, word order and auxiliary verbs (helpers) are more important.
English has these cases for nouns and pronouns: "common case" and "genitive" (possessive).p202 Each may take a plural:
English adjectives unchanged: "red hat", "red hats".