Indefinite pronouns refer to nonspecific persons or things. Most indefinite pronouns express the idea of quantity and share properties of collective nouns (see 7.8.5, Subject-Verb Agreement, Collective Nouns).
Some indefinite pronouns (eg, each, either, neither, one, no one, everyone, someone, anybody, nobody, somebody) always take singular verbs; some (eg, several, few, both, many) always take the plural; and some (eg, some, any, none, all, and most) may take either the singular or the plural, depending on the referents. In the last case, usually the best choice is to use the singular verb when the pronoun refers to a singular word and the plural verb when the pronoun refers to a plural word, even when the noun is omitted.
Some of her improvement is due to the increase in dosage.
Some of his calculations are difficult to follow.
Most of the manuscript was typed with a justified right-hand margin.
Most of the manuscripts are edited electronically.
Some of the manuscripts had merit, but none was of the caliber of last year’s award winner.
None of the demographic variables examined were found to be significant risk factors.
The use of an indefinite pronoun as the antecedent of another pronoun can create confusion. Some writers try to avoid gender bias by using their or he/she with plural indefinite pronouns (eg, Everyone should cite their sources.). Grammatically, the use of a plural pronoun is not correct, and constructions such as s/he are distracting. He or she should be used consistently and the writer should keep the context in mind when making these decisions. Better still, when possible use the plural throughout3 (eg, All authors should cite their sources.). (See also 11.10.1, Correct and Preferred Usage, Inclusive Language, Sex/Gender.)