The subjective case is the case used for a noun or pronoun which is the subject of a verb. For example (subjective case shaded):
Lee eats pies.
(The noun Lee is the subject of the verb eats. Lee is in the subjective case.)
He eats pies.
(The pronoun He is the subject of the verb eats. He is in the subjective case.)
They eat pies.
(The pronoun They is the subject of the verb eats. They is in the subjective case.)
The subjective case is also used for a subject complement. For example: