Learn Croatian: Jesi li zadrhtao kada si vidio medvjeda? - Did you start trembling when you saw the bear?

The base verb drhtati is imperfective and means to tremble or shiver continuously. Adding the prefix za- creates the perfective verb zadrhtati, which expresses the sudden inception or start of an action. Here, it means you suddenly shuddered or started to tremble the moment you saw the bear.

Why do we use the perfective verbs 'zadrhtati' and 'vidjeti' rather than their imperfective equivalents?

Both actions are framed as sudden, completed events in the past. You spotted the bear in a single instant (vidio), which immediately triggered a sudden shudder (zadrhtao). If the sentence meant 'Were you trembling while you were looking at the bear?', it would use the imperfective forms: Jesi li drhtao kada si gledao medvjeda?

Why is the object 'medvjeda' instead of 'medvjed'?

medvjed (bear) is a masculine noun. Remember that for masculine nouns representing living beings (animate nouns), the accusative case takes the same ending as the genitive case, which is -a. If the object were an inanimate masculine noun, the accusative would be identical to the nominative.

Why does the sentence start with 'Jesi li' instead of just using the short auxiliary 'si'?