This is the 477th most frequent Slovak word.
ktorou
"Which" or "by which" (instrumental case, feminine form).
'ktorou' is used here as an instrumental relative pronoun referring to 'knihu' (book), serving as the object of the preposition 's' (with).
Dostal novú knihu, s ktorou teraz radšej trávi večery.
He received a new book, with which he now prefers to spend his evenings.
'ktorou' is an instrumental relative pronoun referring to 'otázka' (question), connected to the preposition 'na' indicating the focus object.
Otázka, na ktorou sa sústredí, je zložitá.
The question on which he is focusing is complicated.
'ktorou' functions as an instrumental relative pronoun describing 'cesta' (path), indicating the means of our journey.