Learn Hardcore Croatian: Lijek koji doktor daje je dobar. - The medicine that the doctor gives is good.

Because the relative pronoun is the direct object of the verb in the relative clause, and it refers to an inanimate masculine noun. For masculine inanimate nouns, the accusative singular has the same form as the nominative: koji. If the antecedent were masculine animate (e.g., a man), the accusative would be kojeg.

Inanimate: Lijek koji doktor daje … (medicine that the doctor gives)
Animate: Čovjek kojeg doktor vidi … (the man whom the doctor sees)

What case is koji here, and what does it agree with?

Case: Accusative singular (object of daje in the relative clause).
Agreement: It agrees in gender and number with its antecedent lijek (masculine singular). The case is determined by its function in the relative clause (object).

Why is there no word for “the” (as in “the medicine”)?

Croatian has no articles. Definiteness (“the”) is understood from context, word order, and modifiers. If you need to make it explicit, you can add a demonstrative: Taj lijek koji doktor daje je dobar (That medicine … is good).

Why is je placed after the whole subject phrase and not right after Lijek?

Je is a clitic (an unstressed short form of biti, “to be”) and must appear in the second position of its clause—after the first complete, stressed constituent. Here, the first constituent is the entire subject noun phrase Lijek koji doktor daje, so je comes after it: Lijek koji doktor daje je dobar. You cannot split the subject phrase with a clitic.

Can I move parts around? What word orders are also natural?

Yes, Croatian allows some flexibility for emphasis, while keeping clitic placement rules:

Lijek koji doktor daje je dobar. (neutral)
Dobar je lijek koji doktor daje. (emphasizes “good”)
Lijek koji doktor daje dobar je. (also possible; prosodically puts focus at the end) Avoid: Lijek je koji doktor daje dobar (ungrammatical: clitic splits the subject phrase).

Why is it dobar, not dobro or dobra?

The predicate adjective agrees with the subject lijek in gender, number, and case. Lijek is masculine singular, so you use dobar (masc. sg.). Compare:

Feminine: Knjiga … je dobra.
Neuter: Pismo … je dobro.
Plural masculine: Lijekovi … su dobri.

What form is daje? Is it from dati or davati?

Daje is 3rd person singular present of davati (imperfective “to give” used for ongoing/habitual actions). The perfective partner is dati (“to give” as a single, completed event). Examples:

Habitual/ongoing: Doktor daje lijek.
Single completed event (past): Doktor je dao lijek.

Could I say što instead of koji?

In everyday speech, yes: Lijek što doktor daje je dobar is common. In careful/standard writing, koji is preferred as the relative pronoun. Also note:

With prepositions and in oblique cases, koji is standard (e.g., lijek za koji …). Using što there is either not possible or very colloquial.
With što, some dialects add a resumptive pronoun: Lijek što ga doktor daje … (colloquial). With koji, you do not add ga.

Why isn’t there an explicit object after daje in the relative clause?

Because koji itself is the object of daje within the relative clause. It stands in for lijek. Adding a pronoun like ga would be redundant and, with koji, considered incorrect: not koji ga doktor daje.

What case is lijek, and why?

Lijek is nominative singular (masculine) because it is the subject of the main clause. The predicate adjective dobar is also nominative masculine singular to agree with it.

What case is doktor here?

Doktor is nominative singular because it is the subject of the relative clause (doktor daje). If its role changed, the case would change (e.g., doktora as genitive/accusative, doktoru as dative).

Can I drop the relative pronoun, like in English “the medicine the doctor gives”?

No. Croatian does not allow dropping the relative pronoun in such clauses. You need koji/koja/koje (or colloquial što).

How would this look in the past tense?

You add the auxiliary je inside the relative clause, still obeying clitic placement:

Lijek koji je doktor dao je dobar. Here, the first je belongs to the relative clause (second position there), and the second je is the main-clause copula (second position after the subject phrase).

When do I use commas around the relative clause?

Restrictive (identifies which medicine): no commas: Lijek koji doktor daje je dobar.
Non-restrictive (extra info): commas: Lijek, koji doktor daje, je dobar. In the non-restrictive version, you presume the listener already knows which medicine; you’re just adding information.

How do I pronounce j in lijek, daje, je?

Croatian j is like English “y” in “yes.”

lijek ≈ “lyek” (Ijekavian: the ije is a single long vowel sequence)
daje ≈ “DA-ye”
je ≈ “ye”

What changes with a different gender or number of the antecedent?

The relative pronoun and the predicate adjective change to agree:

Feminine: Knjiga koju doktor daje je dobra.
Neuter: Pismo koje doktor daje je dobro.
Plural masculine: Lijekovi koje doktor daje su dobri. Note the plural verb su.

Is doktor the only word for “doctor”?

No. Liječnik/liječnica is also common (often more formal). Your sentence works with either:

Lijek koji liječnik daje je dobar.
Lijek koji doktor daje je dobar.