Learn Croatian: Onaj vojnik ukrao je moje pivo, seljače. - That soldier stole my beer, peasant.

Because the speaker is talking directly to the peasant, the noun must be in the Vocative case. For most masculine nouns, we add -e to form the Vocative. However, when a noun ends in -k (like seljak), the k changes to č before the -e. This sound change is called palatalization!

The word vojnik also ends in -k. Why didn't it change to vojniče?

Palatalization only happens in specific situations, like forming the Vocative case. Here, the soldier is the subject of the sentence (the one doing the stealing), so vojnik is in the Nominative case and keeps its regular -k. If you were addressing the soldier directly, it would change to vojniče.

Why does the verb ukrao end in -o instead of a regular past-tense -l?

The infinitive is ukrasti (to steal). The masculine singular past tense form used to end in -l (ukral), but in Croatian, an l at the very end of a syllable usually transforms into an o. Since it matches the masculine noun vojnik, it becomes ukrao. If the soldier were female, it would be ukrala, keeping the l because adding -a means it is no longer at the end of the syllable.