Новая грамматическая тема: относительные предложения с «который» после предлогов — «в котором», «с которой», «у которой», «о котором».

It introduces a relative clause: a clause that gives more information about a noun.

In English, this is often who, which, or that:

• дом, в котором я живу = the house that I live in
• женщина, с которой я работаю = the woman who I work with

So который is the Russian word that connects the main noun to the extra description.

Because in Russian, the preposition stays directly with который:

• в котором
• с которой
• о котором
• у которой

English often allows the preposition to move to the end:

• the house that I live in
• the person I spoke with

Russian normally does not do that. You should say:

• дом, в котором я живу

not:

• дом, который я живу в

So the safe rule is: preposition + the correct form of который.

Because two things affect the form of который:

1. The preposition chooses the case
2. The noun being described chooses the gender and number

For the prepositions in your topic:

• в
  • prepositional → в котором / в которой / в которых
• о
  • prepositional → о котором / о которой / о которых
• с
  • instrumental → с которым / с которой / с которыми
• у
  • genitive → у которого / у которой / у которых

So the ending is not random. It tells you both:

• what case is needed after the preposition
• what noun который refers to

It agrees with the noun it refers back to, not with some other noun inside the relative clause.

For example:

• девушка, у которой есть брат

Here которой is feminine because it refers to девушка, not to брат.

Another example:

• дом, в котором есть магазин

Here котором is masculine because it refers to дом.

So always ask: What noun is being described?
That is the noun that controls the gender and number of который.

It can correspond to all of them.

Russian does not split this up the same way English does. The same word family который can refer to:

• people: человек, с которым...
• things: фильм, о котором...
• places: город, в котором...

So when you see который, do not think of only which. In relative clauses, it can also be who or that, depending on the noun.

This is a very common point of confusion.

In Russian, у + genitive is often used to show possession:

• у женщины есть машина = literally at the woman there is a car
• natural English: the woman has a car

So:

• женщина, у которой есть машина

means:

• the woman who has a car

Here у которой does not mean only a physical location like by whom. Very often it means something like who has.

Sometimes, yes, but not always.

For places, both can be possible:

• дом, в котором я живу
• дом, где я живу

Где is often more conversational and simpler.
В котором is more explicit and works well in formal or careful speech.

But где only works for location-type meanings. It cannot replace other preposition patterns:

• с которой → not где
• у которой → not где
• о котором → not где

So где is only a partial substitute, mainly for place.

Usually no.

English can say:

• the book I told you about
• the woman I work with

Russian normally needs an explicit linking word:

• книга, о которой я тебе говорил
• женщина, с которой я работаю

Also, Russian normally does not leave the preposition stranded at the end. So for learners, the best rule is:

• do not omit который
• do not move the preposition away from it

Yes, normally you do.

Russian relative clauses with который are usually separated by commas:

• дом, в котором я живу
• человек, с которым я говорил
• книга, о которой мы спорили

If the relative clause is in the middle of the sentence, it is set off with commas on both sides.

This is an important difference from English, where comma use depends more on whether the clause is restrictive or non-restrictive. In Russian, with который, you normally write the comma.

A good step-by-step method is:

1. Start with the noun you want to describe
   
   • дом
2. Decide what relationship the clause has
   
   • live in → в
3. Choose the right case after the preposition
   
   • в → prepositional
4. Make который match the noun
   
   • дом is masculine singular → котором
5. Add the rest of the clause
   
   • дом, в котором я живу

More examples:

• женщина, с которой я работаю
• студент, у которого есть вопрос
• фильм, о котором все говорят

So the basic pattern is:

noun + comma + preposition + correct form of который + rest of clause