Indefinite pronouns are used in those cases where the subject or person can not be determined exactly or not at all. Most indetfinite pronouns are derived from the interrogative pronouns by the particles -то, -либо, -нибудь, кое-, e.g.: кто́-то, что́-либо, где́-нибудь, кое-кто́.
The particle -то is most commonly used, especially in colloquial Russian, i.e. кто-то, что-то, чей-то, кого-то, чему-то, с чьим-то etc. In the literary language, this particle is used when it comes to an object/person that really exists, but for the speaker or his companion is unknown:
The particle -нибудь is used with respect to any subject/person, most often this particle is used in phrases of conditional, queries, commands, notification phrases with shapes of future time:
The particle -либо is a synonym of the particle -нибудь, mainly used in scientific and official business style.
Examples of the National Corpus of Russian Language:
The particle кое- is used in those cases where the speaker does not want to name the object/person he /she speaks:
The indefinite pronouns of the type, such as чей-то, кто-нибудь, что-то, кое-кто are declinable, and the particles do not change, e.g.: кто-нибудь, кого-нибудь, кому-нибудь etc.; кое-кто, кое-кого, кое-кому etc.
In the case of a particle кое-, we must pay attention to the forms of Locative and the Instrumental where the so-called broken forms occur: кое о ком, кое с кем, кое о чём, кое с чем.
Pronouns некто, нечто are indeclinable and are used in written language.
Examples of the National Corpus of Russian Language:
Other examples of indefinite pronouns that are actively used in colloquial are: