This is the 12716th most frequent Russian word.
которій
"Которій" is an outdated form of "который," meaning "which" in English.
Here, 'которая' is the feminine singular form of 'который', matching the gender and case of 'книга'. It links the noun with the qualifying predicate.
Я прочитал книгу, которая оказалась интересной.
I read a book that turned out to be interesting.
In this example, 'которому' is the dative singular form of 'который', reflecting its use in a relative clause that describes the noun 'человек'.
Это человек, которому я доверяю.
This is the person whom I trust.
In this sentence, 'который' is used as a relative pronoun connecting the noun 'дом' with the clause 'был построен в прошлом году', providing additional information about the house.