Learn Croatian: Jedino što želim jest mir. - The only thing I want is peace.

While samo is an adverb meaning "only" or "just" (e.g., Samo želim mir - I just want peace), jedino is an adjective that is acting as a noun here, meaning "the only thing." Because it acts as a "thing," we can attach the describing clause što želim to it.

Why use što to mean "that" instead of a form of koji?

When "that" or "which" refers back to an abstract concept, an indefinite pronoun (like sve - everything, or nešto - something), or a neuter adjective acting as a noun (like jedino - the only thing), Croatian uses što. You use koji when referring back to a specific, concrete noun.

Why is the verb jest used here instead of the standard je?

Jest is the long, stressed form of je (is). In written or formal Croatian, this long form is preferred when linking a whole clause ("the only thing I want") to a noun ("peace") in an "A is B" equation. In casual everyday speech, however, you will often hear people just use the short form je.

Why is mir in the nominative case? Shouldn't it be accusative since it is the thing being wanted?