Learn Croatian: Što se dogodilo jučer? - What happened yesterday?

In Croatian, the question word što (what) is treated as a neuter singular noun. Because of this, the past tense verb describing its action must take the neuter ending -o. This rule also applies to indefinite pronouns you are learning now, like nešto (something) or ništa (nothing).

Why do we need the word se?

The verb meaning "to happen" is dogoditi se. It is a reflexive verb, which means it naturally comes with the word se. You can't just use the verb on its own; it must always be paired with se.

Can I change the word order to Što dogodilo se jučer?

No. The word se is a small, unstressed word (a clitic). In Croatian, clitics strictly want to be in the "second position" of the sentence. Here, it steps right behind the first word, Što.

How would I change this sentence to say "Something happened yesterday"?

You just replace the question word što (what) with the indefinite pronoun nešto (something), making it: Nešto se dogodilo jučer. Notice how the verb stays exactly the same, since nešto also requires a neuter verb!