GrammatikInterrogativ / Relativ

This small family of words - кто, что, како́й, кото́рый, чей, ско́лько - does two things. First, they ask questions. Second, they join two clauses together as relative words. Have a look:

• Asking: Кто э́то? (Who is this?)
• Joining: Я зна́ю, кто э́то. (I know who this is.)
• Asking: Кто написа́л э́ту кни́гу? (Who wrote this book?)
• Joining: Тот, кто написа́л э́ту кни́гу… (The one who wrote this book…)

When they open a question, we call them interrogative. When they link a subordinate clause to the main sentence, we call them relative.

кто and что: who and what

кто asks about people (and animals), что asks about things. Both are singular and both decline through all six cases, but they have no gender or number of their own.

The case is chosen exactly as for any noun, by the verb or preposition in front of it:

• Чего́ ты бои́шься? (What are you afraid of?) — genitive after боя́ться
• Кому́ ты позвони́л? (To whom did you call?) — dative
• Кого́ ты ви́дишь? (Whom do you see?) — accusative object
• С кем ты идёшь? (With whom are you going?) — instrumental after с
• О чём вы говори́те? (What are you talking about?) — prepositional after о

Verb agreement: masculine with кто, neuter with что

This is a fixed rule and easy to do wrong. In the past tense and for any adjective кто always behaves like a masculine singular word, and что always behaves like a neuter singular word — no matter who or what the real answer is:

• Кто пришёл? (Who came?) — masculine, even if the answer is a woman.
• Что случи́лось? (What happened?) — neuter.
• Кто гото́в? (Who is ready?) — the adjective is masculine (гото́в).
• Что гото́во? (What is ready?) — the same adjective turns neuter (гото́во).

So remember: кто → masculine, что → neuter.

чей: whose

чей asks about possession. Unlike кто and что, it agrees with the possessed thing in gender, number and case, just like the possessive pronouns мой, твой:

It declines like a soft possessive pronoun (the same soft endings as мой) through all six cases:

It also works as a relative word: писа́тель, чьи кни́ги я люблю́ (the writer whose books I love), though in everyday speech you would rather use кото́рого here.

како́й: what kind / which

како́й asks about the quality or type of something. It declines like an adjective and agrees with its noun in gender, number and case (see adjective declension):

It answers “what is it like?”: — Кака́я э́то маши́на? — Кра́сная и бы́страя. (— What kind of car is it? — Red and fast.)

кото́рый: the main relative pronoun

кото́рый means which, who, that and is the default relative pronoun: "the woman who…", "the book that…", "the house in which…".

It declines like an adjective, with two rules:

• Gender and number come from the noun it refers back to in the main clause.
• Case comes from the role кото́рый plays inside its own clause, not from that noun.

That sounds difficult but actually is quite easy!

The case logic

Look at the same noun, three different clauses. The gender/number stays feminine singular (from же́нщина / кни́га), but the case changes with the job inside the clause:

A good test: cover the main clause and turn the relative clause into its own little sentence. “…she lives here” → subject → nominative кото́рая. “I am reading it” → object → accusative кото́рую. The gender you already know from the noun it refers back to; the case you find from this mini-sentence.

More worked examples across genders:

• Друг, кото́рого я жду… (The friend whom I am waiting for…) — masc., genitive after ждать.
• Учи́тель, кото́рому я написа́л… (The teacher to whom I wrote…) — masc., dative.
• Го́род, в кото́ром я роди́лся… (The city in which I was born…) — masc., prepositional.
• Лю́ди, кото́рые живу́т ря́дом… (The people who live nearby…) — plural, nominative.
• Пробле́ма, о кото́рой мы говори́ли… (The problem we talked about…) — fem., prepositional.

Note that a preposition sits in front of кото́рый and stays inside the relative clause: стол, на кото́ром лежи́т кни́га (the table on which the book lies).

The comma before кото́рый is obligatory in writing.

како́й vs кото́рый

Both can come out as which in English. The textbook distinction is:

• како́й asks about quality or type: what kind?
  • Како́й суп ты хо́чешь? (What kind of soup do you want?)
• кото́рый picks one out of a known set: which one (of these)?
  • Кото́рый из них твой? (Which one of them is yours?)

In normal life, though, use како́й for both. Russians normally use како́й even for “which one of a set?”, and кото́рый as a question word sounds formal or bookish:

• Кака́я из э́тих су́мок твоя́? (Which of these bags is yours?) — the everyday wording
• В како́й день? (On which day?) — the everyday wording
• Кото́рый из них твой? (Which one of them is yours?) — correct, but sounds formal

The one spot where кото́рый still shows up as a question is telling the time: Кото́рый час? (What time is it? — literally “which hour?”). Even there, the colloquial version is Ско́лько вре́мени? (literally “how much time?”).

So in practice: use како́й when you ask a question, and кото́рый as the relative pronoun that joins clauses (тот, кото́рый… (the one that…)), often paired with the demonstratives тот, э́тот.

ско́лько: how many / how much

ско́лько asks about quantity and takes the noun that follows it in the genitive — genitive plural for countable things, genitive singular for uncountable ones:

• Ско́лько книг у тебя́? (How many books do you have?) — gen. plural
• Ско́лько студе́нтов в гру́ппе? (How many students are in the group?) — gen. plural
• Ско́лько воды́? (How much water?) — gen. singular
• Ско́лько вре́мени? (How much time? / What time is it?) — gen. singular
• Ско́лько э́то сто́ит? (How much does this cost?) — no noun needed

The genitive-after-quantity rule is the same one used with numbers; see the details in the genitive article. ско́лько itself can decline (ско́льким, ско́лькими), but the nominative form covers most everyday questions.

Indirect (embedded) questions

The interrogatives keep the very same forms when a question is reported or embedded inside a bigger sentence, typically after verbs like знать (to know), спроси́ть (to ask), по́мнить (to remember). A comma separates the two parts, and word order stays normal; there is no inversion as in English:

• Я не зна́ю, что де́лать. (I don’t know what to do.)
• Он спроси́л, кто я. (He asked who I was.)
• Скажи́, кака́я сего́дня пого́да. (Tell me what the weather is like today.)
• Я не по́мню, ско́лько э́то сто́ит. (I don’t remember how much it costs.)
• Она́ зна́ет, чей э́то дом. (She knows whose house it is.)

These are not relative clauses (they don’t describe a noun), they are questions in disguise, which is why they still use the question words rather than кото́рый.

Common mistakes

• Giving кото́рый the case of the noun it refers back to. The case comes from the role inside the relative clause.
  Кни́га, кото́рая я чита́ю (wrong) → Кни́га, кото́рую я чита́ю (correct — object → accusative)
• Wrong verb agreement with кто / что. кто is masculine, что is neuter.
  Кто пришла́? (wrong) → Кто пришёл? (correct)
• Confusing како́й and кото́рый. Use како́й for “what kind”, кото́рый to pick one from a set.
  Кото́рый суп ты хо́чешь? (odd) → Како́й суп ты хо́чешь? (correct)
• Nominative after ско́лько. The counted noun goes in the genitive.
  Ско́лько кни́ги? (wrong) → Ско́лько книг? (correct)
• Forgetting that чей agrees. It matches the possessed noun, not the owner.
  Чей маши́на? (wrong) → Чья маши́на? (correct)

Summary

• The same words ask questions and join clauses as relatives.
• кто (people) and что (things) decline through all cases; кто takes a masculine verb, что a neuter one.
• чей and како́й agree with their noun in gender, number and case.
• кото́рый takes its gender/number from the noun it refers back to but its case from its role inside the relative clause.
• како́й = what kind; кото́рый = which one of a set (and the main relative pronoun).
• ско́лько is followed by the genitive.