Breakdown of Тренер даёт свою книгу чемпиону, которому он доверяет.
книга
book
он
he / it (for a masculine noun) / it (for a neuter noun)
давать
to give (imperfective)
свой
one's own
Questions & Answers about Тренер даёт свою книгу чемпиону, которому он доверяет.
Why is которому in the Dative case here?
Которому is the Dative masculine form of который (which/who). We use the Dative case here because the verb in the second half of the sentence, доверяет (trusts), demands it. In Russian, you trust 'to' someone, so the person being trusted takes the Dative case.
Does которому match the case of чемпиону because they refer to the same person?
Not exactly! It is actually a coincidence here. The word которому gets its gender and number (masculine singular) from the noun it refers back to, чемпиону. However, it gets its case based on its job in its own part of the sentence. It just so happens that giving to someone (Dative) and trusting someone (Dative) both need the same case here.
Why does the sentence use свою книгу instead of его книгу for 'his book'?
Свою comes from the reflexive possessive pronoun свой, which means 'one's own'. Because the subject of the sentence (the coach) is the owner of the book, you have to use свой. If you used его (his), it would mean the coach is giving some other man's book to the champion.
Can we skip которому and just say чемпиону он доверяет, like in the English translation?