Did you know that Russian indefinite pronouns can have two particles, то and нибудь, and that they are used in different contexts?
The pronouns with the particle нибудь are always indefinite. They are usually used in questions, commands and statements in the future. They are also used in the present when they mean something indefinite.
Ты хочешь что-нибудь? Do you want anything?
Она куда-нибудь поедет? Is she going anywhere?
Кто-то is definite but not yet defined. It is usually used in statements in the past.
Тебе кто-то звонил. Somebody called you.
Что-то случилось. Something happened.