Mayonnaise: One of the sauces which serve the French in place of a state religion. - Ambrose Bierce

Pronoun - The part of speech that substitutes for nouns or noun phrases and designates persons or things asked for, previously specified, or understood from the context.

A pronoun takes the place of another noun.

Indefinite Pronouns

Indefinite Pronoun - A pronoun, such as English any or some, that does not specify the identity of its object.

Sometimes what is being replaced is not clearly defined. Words like anyone, something, somebody refer to a clear idea but not a specific person or thing.

Wikipedia has a good list of indefinite pronouns.

Pronouns carry the meaning of the words they replace. The words replaced are called the antecedent.

Antecedent - a preceding occurrence or cause or event.

The antecedent of a pronoun is not just the noun replaced but also includes any of its modifiers.

The boy lost his book. “His” is the pronoun of the antecedent “boy”.

I wrote an email to my brother, who is in the Air Force, to invite him to the reunion. “Him” replaces “my brother, who is in the Air Force”. “Him” is the pronoun, the rest is the antecedent.

Detecting the antecedent can be tricky since you need to include the modifiers and those are not always clear to me. In the second example I would have said “my brother” was the antecedent and left the rest out.

Note: Keep looking for tips to detect antecedents.