Quick answer
Како́й goes with a noun and agrees with it. Как goes with an adjective or adverb and never changes.
Exclamations
| Russian | English | Why |
|---|---|---|
| Кака́я краси́вая у́лица! | What a beautiful street! | noun follows |
| Как краси́во! | How beautiful! | adverb, no noun |
| Како́й у́мный ма́льчик! | What a clever boy! | noun |
| Как хорошо́! | How good! | adverb |
| Кака́я жа́лость! | What a pity! | noun |
If you can put a noun after it, use како́й and agree it. If there is no noun, use как.
Questions
Како́й asks about a quality or an identity, кото́рый about a position in a sequence: Кото́рый час? — what time is it (literally “which hour”). That is why the time question uses кото́рый and not како́й.
The emotional ones
Како́й у́жас! — how awful. Кака́я пре́лесть! — how lovely. Как жаль! — what a shame. Как ми́ло! — how sweet. These are complete utterances and extremely common; learning five of them gives you a natural-sounding reaction for most situations.
Что за…?
A third option, informal and slightly indignant: Что э́то за кни́га? — what sort of book is this? The noun stays in the nominative and за does nothing to it, which is unusual enough to be worth noting.
Typical mistakes
Как краси́вая у́лица!
Кака́я краси́вая у́лица!
A noun follows, so како́й.
Како́й хорошо́!
Как хорошо́!
An adverb follows, so как.
Check yourself
Which is correct: “How beautiful!” with no noun?