The concept of cases such as nominative and accusative, etc. is actually familiar to English speakers, although many are often not conscious of it. Note, for example, how our nominative pronouns “he” and “she” change to “him” and “her” when they are used in the accusative case. If you would like more explanation of the concept of cases (or other grammatical concepts), it often helps students to review English grammar using any English grammar reference book.
In German, just as in English, the accusative case is used primarily for the direct objects of sentences. For example, in “They hit the ball,” the direct object is “the ball.” The German definite article changes in accusative case only for those direct objects which are masculine, as the following chart indicates:
Our sentence in German then is: Sie schlagen den Ball (They hit the ball). In vocabulary lists you will often see that Ball is listed as der Ball, which is its nominative-case singular form.