Breakdown of Oštećeni proizvod što sam im ga poslao ima jamstvo.
biti
to be (sam, si, je)
on
he
oni
they
imati
to have
Questions & Answers about Oštećeni proizvod što sam im ga poslao ima jamstvo.
Why is the word ga needed in this sentence?
Because we are using the unchanging word što to mean 'that', the sentence needs a way to show the grammatical case of the product being sent. We use the accusative clitic pronoun ga (meaning 'it') to show that the product is the direct object of poslao (sent).
Could I use koji instead of što here?
Yes, that is the standard way to form this relative clause. You would say proizvod koji sam im poslao. Notice that when you use koji, it changes form to show the accusative case itself, so you must drop the extra ga. Using što plus a resumptive pronoun is a very common alternative, especially in spoken Croatian.
Why are the three short words ordered as sam im ga?
Croatian clitics (short, unstressed words) must follow a strict sequence. The verbal auxiliary sam comes first. After that, dative pronouns always precede accusative pronouns, so the dative im (to them) must come before the accusative ga (it).
Why does the sentence use the form Oštećeni instead of just Oštećen?
Oštećeni is the definite form of the adjective. Because the sentence refers to a specific, known item ('the damaged product') which is then uniquely identified by the relative clause, the definite form is required here.