кто-то vs кто-нибудь

kto-to vs kto-nibud' — someone specific vs anyone/someone

What it really means

Кто-то is an unspecified but particular person, often in statements. Кто-нибудь is open-ended “anyone/someone,” common in questions, offers, and future possibilities.

Examples in natural speech

Кто-то звонит.
Someone is calling.
Я видел кого-то знакомого.
I saw someone familiar.
Кто-нибудь знает ответ?
Does anyone know the answer?
Позови кого-нибудь.
Call someone.
Если кто-нибудь придёт, скажи мне.
If anyone comes, tell me.

Common mistakes

Do not automatically use кто-то in questions; кто-нибудь is usually more natural for an open question.

Memory tip

-то points to a particular unknown; -нибудь leaves the choice open.

Frequently asked questions

Which in questions?

Usually кто-нибудь.

Which means a specific unknown person?

Кто-то.

What case changes?

Both decline: кого-то, кому-нибудь.

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