Indefinite pronouns replace nouns whose identity is undetermined or varies. Common indefinite pronouns include everyone, anything, somebody, no one, and whoever.
In 1877, no one could imagine following the individual
histories of thousands of gas molecules, and so Boltzmann opted for following
--Donald M. McCloskey, "Computation Outstrips Analysis" (modified)
Anyone who has at least a 90% average at the end of the
semester does not have to take the final examination.
Check the verb agreement carefully when using an indefinite pronoun as a subject. Indefinite pronouns often contain quantifiers, which have special rules for subject-verb agreement.