The antecedent of a pronoun is the noun, noun phrase or noun clause being replaced by the pronoun.

• I bought some batteries, but they were not working.
  (‘some batteries’ is the antecedent of the pronoun ‘they’).
• The girl, whom you met yesterday, was my sister.
  (‘girl’ is the antecedent of the pronoun ‘whom’).

Pronouns can be classified in three cases:

• Subjective Case
• Objective Case
• Possessive Case