Modern English has only three cases, the subjective/nominative, objective/accusative and possessive/genitive, plus some vestiges of the dative. Additionally, these only mostly apply to pro-forms (i.e. pronouns and possessive determiners).
Other Germanic languages tend to have more cases, and historical linguistics shows that the case system in English has been significantly simplified. Traditional grammar often places an emphasis on case because it was derived from Latin grammar, and Latin has a significant case system. In modern grammars however, its role is much diminished.