A pronoun is a word that substitutes for a noun or a noun phrase. Pronouns are classified into 6 types: personal pronouns; demonstrative pronouns; indefinite pronouns; relative pronouns; interrogative pronouns; and reflexive and intensive pronouns.

Indefinite pronouns refer to unspecified people or things. These pronouns can be singular or plural; some are always singular, some are always plural, and the number of some depends on the object of the preposition associated with the pronoun.

These are singular indefinite pronouns:

• each, either, neither, one
• no one, nobody, nothing
• anyone, anybody, anything
• someone, somebody, something
• everyone, everybody, everything

A singular indefinite pronoun takes a singular verb.

• Everyone is happy.
• No one knows what to do in this situation.

When a singular indefinite pronoun serves as an antecedent in a sentence, it must take a singular pronoun referent.

• Everyone brought his book.
• No one forgot her homework.

These are plural indefinite pronouns:

• several, few, both, many

A plural indefinite pronoun takes a plural verb.

• Both are acceptable.
• Few are going to the party.

When a plural indefinite pronoun serves as an antecedent in a sentence, it must take a plural pronoun referent.

• Both passed their tests.
• Few did not understand their assignments.

These are indefinite pronouns that can be singular or plural depending on how they are modified by a prepositional phrase:

• some, any, most, all, none (Use this mnemonic device: SAMAN)

The number of the verb or the pronoun referent depends on whether the noun is countable or uncountable.

When the object of the preposition is uncountable, use a singular verb.

• All of my knowledge was useful during the test. (Knowledge cannot be counted: how many knowledge? This does not make sense.)
• None of the ice cream was eaten. (How many ice cream? Ice cream cannot be counted.)

When the object of the preposition is uncountable, use a singular pronoun referent.

• All of her work made its impact on her grades. (Work cannot be counted: how many work?)
• Some of the water left its mark on the wooden table. (How many water? Water cannot be counted.)

When the object of the preposition is countable, use a plural verb.

• Most of the words were on the vocabulary test. (Words can be counted.)
• None of these sentences are hard to understand. (Sentences can be counted.)

When the object of the preposition is countable, use a plural pronoun referent.

• All of the books were useful in their own ways. (Books can be counted.)
• Some of my friends drove their cars to the beach. (Friends can be counted.)

Note: Some indefinite pronouns are identical to the indefinite determiners (also known as "indefinite adjectives"). The part of speech of each word is determined by its function in the sentence.

• Many books are wonderful. [In this sentence, "many" is an indefinite determiner that modifies the noun "books."]
• Many have become my favorites. [In this sentence, "many" is an indefinite pronoun functioning as the subject.]