The children bought some flowers for their mother. The accusative case will usually be translated as a direct object . This is the case of direct address. Son, give me the newspaper. Because word order in Greek can vary , case becomes the primary means of identifying the function of a noun in a sentence, e.g., the subject as opposed to the direct object.
Word order is the key.
Not so in Greek. Both word orders could mean exactly the same thing. The subject is the word in the nominative case and the direct object is the word in the accusative case. Generally, a given noun is either masculine, feminine, or neuter. It will not be masculine in one context, and feminine in another context.