An indefinite pronoun is an article of speech that replaces a specific noun without mentioning what that noun is. The noun in question may be mentioned somewhere else in the sentence, or somewhere else in the context of the writing, usually within the containing paragraph. An indefinite pronoun can be either plural or singular, but no matter what one is chosen, it is important to make sure all other parts of the sentence are in grammatical agreement with the pronoun.
Singular indefinite pronouns include another, anybody, anyone, each, either, nobody, and many others. These indefinite pronouns do not necessarily directly identify the objects they are referencing. For example, saying, "One got lost on the way to town," does not identify what the one thing was that got lost. However, the clue to what the indefinite pronoun referred to should be found somewhere else in the context of the writing. The verb form with such pronouns is singular.
Plural indefinite pronouns include others, both, many, several, few, and many. The verb form, in this case, is plural, even though the subject may appear singular. In some cases, it can be hard for writers to tell the difference, and the construction of the sentence could help cause confusion, so writers should be careful to make sure the subject and verb agree.
Another concept somewhat unique about an indefinite pronoun is that it may also function as an adjective in another context or sentence. For example, consider the sentence, "We will save that for another day." While another can sometimes be an indefinite pronoun, in this particular context it functions as an adjective because it is further identifying or describing a noun. If asked to identify the different articles of speech, it may not always be correct to assume a word always represents a certain thing.
Something worth considering when using indefinite pronouns is whether it is better or more useful to use a plural form rather than a singular form. Using a singular form can sometimes be awkward, as seen in the sentence "Everybody should check his or her own paper." It may be easier and more efficient to say, "All of them should check their own papers."
In some cases, it may be possible for an indefinite pronoun to have both a singular and plural form. These include all, more, most, any, some, and such, among others. As with other situations, the context of the writing should provide enough of a clue to alert the reader whether the subject is plural or singular.