Relative koji in the oblique cases

You already use koji to point back at a noun when it is the subject of its clause (čovek koji radi ovde) or its object (žena koju volim). Now you will describe people the way people really do: the one I told you about, the one I work with, the one I gave the book to. For that, koji has to take every case — and it can stand after a preposition.

Two questions, two different answers

Every form of koji is decided by two separate questions. Beginners mix them up, so keep them apart:

1. Who is it about? Look OUTSIDE the clause, at the noun koji points back to. That noun gives the gender and number: koleginica → feminine singular, ljudi → plural.
2. What does koji do inside its own clause? Look INSIDE the clause, at its verb or preposition. That gives the case: dati nekome → dative, pričati o nekome → locative, raditi s nekim → instrumental.

Take Ovo je koleginica kojoj sam dao knjigu (man speaking). Question 1: koleginica is feminine singular. Question 2: inside the clause I gave the book to her — dative. Feminine + singular + dative = kojoj. The main clause (Ovo je…) has no say in the case at all.

The forms of koji

koji declines like an adjective such as dobar: -og in the genitive, -om/-em in the dative and locative, -im in the instrumental. Masculine and neuter share everything except the nominative and accusative; in the plural, all genders share the genitive, dative, locative and instrumental.

One sentence for each case

The preposition goes in front of koji, never at the end. English says the girl I told you about and the colleague I work with, with the preposition hanging at the end and the relative word dropped. Serbian can do neither: the preposition moves to the front and koji is always there — devojka o kojoj sam ti pričao, kolega s kojim radim. Say the preposition first, then koji.

A person or a thing? In the masculine accusative the old rule from A1 applies: for a person or an animal use kojeg, for a thing use koji. Prijatelj kojeg sam upoznao (a person) but auto koji sam kupio (a thing).

koji already IS the pronoun. koji has no short clitic form, and you never add a clitic for the same person inside the clause: koleginica kojoj sam dao knjigu, not kojoj sam joj dao. Other clitics (sam, si, je, ti, se) still come second — right after koji or after the preposition + koji: o kojoj sam ti pričao, kojeg je Jelena dovela.