какой-то vs какой-нибудь

kakoy-to vs kakoy-nibud' — some particular kind vs any kind

What it really means

Какой-то refers to an unspecified but particular kind or item. Какой-нибудь means any kind, some random one, or one not specified, especially in requests and future contexts.

Examples in natural speech

У него какой-то странный голос.
He has some strange voice.
Я ищу какой-то документ.
I am looking for a certain document.
Выбери какую-нибудь книгу.
Choose any book.
Купи какой-нибудь хлеб.
Buy some bread.
Есть какой-нибудь вопрос?
Is there any question?

Common mistakes

Do not use какой-то when inviting unrestricted choice; say какой-нибудь.

Memory tip

-то is a known-but-unnamed type; -нибудь is any available type.

Frequently asked questions

Which means any?

Какой-нибудь.

Which signals a particular unknown?

Какой-то.

Do they decline?

Yes: какую-то, каким-нибудь.

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