The concept of "cases" is tricky for English learners of German because our language doesn't have them.
German has four cases and they help us to understand the relationship between words in a sentence. N.B. Every sentence contains at least one verb!
Cases can change the words for "the" and "a" as well as endings on adjectives. Do not be afraid of them!
The Nominative and Accusative Cases
The SUBJECT of the sentence is known as the NOMINATIVE case and refers to the person or thing carrying out the verb.
The DIRECT OBJECT of the sentence is known as the ACCUSATIVE case and refers to the person/thing affected by the verb.