Interrogative pronouns taking different case than it's noun

Question

While reading https://en.m.wikibooks.org/wiki/Russian/Interrogative_Pronouns it's not clear to me the following example:

He talks about the girl who works there - Он говорит о девушке, которая работает там.

Why которая is not declined with the prepositional case? Can you give me more examples of this? They also say:

You may notice that какой ('what kind'), который ('what/which'), and чей ('whose'), are adjectives. They are therefore called adjectival pronouns. As such, when they are used in a sentence, they take on the ending appropriate for the gender and number of the noun it refers to, and the case appropriate for where it is in the sentence. This can mean that it takes on a different case to its noun.

Answer 1

A relative clause in Russian is joined by a pronoun, the relative pronoun который "who, which, that". Actually, который is a conjunction, joining a subordinate clause. Который introduces a sentence that modifies the noun in the main clause. For this reason it has to agree in number and gender with the noun it modifies like adjectives. However, unlike regular adjectives, it does not agree with that noun in case; because it doesn't belong to that clause , it reflects the case of its function in the subordinate clause.

Вот мужчина(masculine, singular ), с которым(masculine, singular ) я вчера познакомился .This is the man who(m) I met yesterday

Которым agrees in gender (masculine) and number (singular) with the noun to which it connects the subordinate clause. However, it reflects the case demanded by the preposition с, the instrumental, in the clause it introduces.

Он влюбился в девушку, которая живет напротив.(девушка живет напротив--которая ) He fell in love with the girl who lived across the street.

Мой сын не ладит с ребятами, которые живут в нашем доме.(ребята живут в нашем доме--которые ) My son doesn't get along with the children who live in our house.

Я не читал книгу, о которой вы говорите. (вы говорите о книге --которой.) I didn't read the book you are talking about.

Answer 2

I believe it takes the case of the part of the clause it is in. For example,

Он говорил о девушке, у которой есть много денег.

Here, которой is genitive, because in the second clause у is used.

In your example, которая is in nominative because the girl is the subject of the clause (i.e., she is the one working).

Another example:

Я увидел человека, которому нравится сыр.

Here, it is in the dative, regardless of the fact that accusative is required in the first clause.

Answer 3

I think I got it, but... what exactly defines a clause?

You have a clause when you have a complete thought. Usually, a clause is defined by one (no more) verb with all its actants, i. e. noun-like words that go with the verb, either nouns or adjectives. The role of every noun-like word within the clause is defined by its case and its preposition. What that role means depends on the verb to use. For example, every transitive verb accepts two roles: the subject (in the nominative case) and the object (in the accusative case) of the action. For the verb «кушать», that's the one who eats and what he eats, respectively. For the verb «целовать», that's the one who kisses and the one who is kissed.

E. g. the clause «которая работает там» has one verb, one adverb (a word that is attached to the verb and “modifies” its meaning — see “ad” + “verb”) and one nominal word: «которая». That word is a pronoun, i. e. it has no meaning of its own and stands for whomever you think of, «девушка» in this case; grammatically, this word is quite like an adjective. Since the thing that the nominal word stands for is the subject of the action that the verb means (“to work”), the pronoun is in the nominative case. Just the same, if the first clause («Он говорит о девушке», which has one verb and two nominals) was not there, in the clause «Девушка работает там» the noun «девушка» would be in the nominative case, which I believe is a very natural thing to see.

There are huge theory books (which I didn't read) about the roles of the verbal actants; above is what is necessary to understand the usage of «который». That's also useful to understand the Russian punctuation. It's pretty mechanical, a foreigner can easily learn it better than most natives.