Indefinite Pronouns

An indefinite pronoun refers to an unspecified or unknown person or thing. Common indefinites include any, both, each, either, every, neither, none, one, other, and some. Many of these can also be used as adjectives.

Some are happy to help. [pronoun]
Some people are happy to help. [adjective]

Compound Indefinite Pronouns

Thing, body, or one can be added to some indefinite pronouns to form a compound.

every + thing = everything
some + body = somebody
any + one = anyone

Compound indefinite pronouns are always spelled as one word. However, if the two words that make up a compound are used instead as an adjective and a noun, they should be spelled as two words.

Anyone who was interested could attend. [Anyone is used as a pronoun.]
Any one of those interested could attend. [One is used as a noun and modified by the adjective any.]

Agreement in Number

When used as the subject of a sentence, an indefinite pronoun should agree in number with the verb. (See Verb-Subject Agreement in Number.) Most indefinites are considered singular, except when they stand in for a plural entity. In that case, they should be treated as plural.

Neither wants to go to the park. [singular]
Both want to stay home instead. [plural]

When an indefinite serves as an antecedent, it should agree in number with the pronoun that refers to it.

Everyone needs to finish his or her assignment. [singular]
Few ever achieve their goals. [plural]

For more information, see Pronoun-Antecedent Agreement in Number