Learn Croatian: Veliko carstvo odjednom pade, a oni pobjegoše u planine. - The great empire suddenly fell, and they fled to the mountains.

Pade is the aorist tense (a literary and historical past tense) of the perfective verb pasti (to fall), conjugated in the third-person singular. In everyday spoken Croatian, you would almost always hear the standard perfect tense instead: je palo (since carstvo is a neuter noun).

How is the verb form pobjegoše constructed?

This is the third-person plural aorist of pobjeći (to flee). For many perfective verbs ending in -ći, the aorist is formed by taking the root (which often ends in a velar consonant like -g or -k) and adding specific aorist endings. Here, the root pobjeg- takes the third-person plural aorist ending -oše.

The English translation says "and they fled", so why does the Croatian sentence use a instead of i?

The conjunction a is used here because there is a change of subject and a slight shift in the narrative. The first clause is about the empire (carstvo), and the second switches to the people fleeing (oni). While i connects things that share a subject or are purely additive, a shifts the focus to a different actor or introduces a mild contrast, even though we usually translate it simply as "and" in English.

Why is planine in the accusative case here rather than the locative case?