Nominatives are the subject of a sentence or a predicate nominative.
In the pattern ‘X sees Y’ or any other sentence pattern that denotes an action with a doer and a reciever, ‘X’ is the nominative.
The nominative in predicate nominatives follows the pattern ‘X is Y’ or any other pattern that equates two or more nouns. Both objects are nominative; X is the subject and Y is the predicate nominative.
Accusatives are direct objects–those that directly recieve the action. Sentences like ‘X hit Y’ and ‘X loves Y’ have X as the nominative and Y as the accusative. Y recieves the action and X is the doer of the action.
Genitives show relationships between nouns. In the pattern ‘X’s Y’, ‘X’ is the genitive and Y is that which is possessed.
If this confuses you, blame English’s mangling of its once-glorious case system.