the part of the sentence about which something is divulged; it is what the sentence's other words are gossiping about

uses the nominative case

The predicate is the other necessary  part of the sentence (along with the subject), the part that has something to say about the subject, that states it's predicament.

Complete predicate: 

The girl is squatting under the bridge.

The simple predicate is just the verb. 

answers the question of what? or whom? after the verb

I scratched the knee.

I threw John the ball.

You gave me a cold. 

Uses the accusative case

tells to or for whom or what the action of the verb, however welcome or unwanted is committed

I threw the boy a ball.

You gave me a cold.

I gave her a piece of my mind.

Uses the dative case

comes hot on the heels of a linking verb to explain or identify the subject

My son is a good student.

My daughter is happy.

uses the nominative case

The loaded dotard live in squalor.

The pendulum swung over the pit.

Tuck yourself in between Mog and me.

Rome is in Italy.

Italy is in Europe.

Objects of prepositions will be in either the ablative or accusative case.  Which depends on the preposition and its meaning in the sentence.

indicates ownership

my thoughts

Amanda's book

Donald's computer

Stephen's game

uses the genitive case

direct address

(person being spoken to)

Timothy, come here.

Amanda, go to bed.

uses vocative case