Nominative vs Accusative vs Dative
Nominative = Subject (Who does the action) Accusative = Direct Object (Who is the action done unto) Dative = Indirect Object (to/for whom is the action done)
In English, you can think of the different types of pronouns for a gender, eg. He, Him.
He does something. = "He" is the nominative Something is done to him. = "Him" is the accusative. They do something for him. = "Him" is the dative. They is the nominative and something is the accusative.
Just like you have to change the case of the pronoun in english (Him does something makes no sense, for instance), you have to do it in German.
While doing your practices, try to look at the role of each noun in the sentence so that it is easier to memorize the case endings.
These two sites have helped me a lot: http://www.german-database.supanet.com/page13.html http://www.acampitelli.com/explain_case_in_German.htm