Learn Croatian: Prvo oguli tri krumpira. - First peel three potatoes.

After the numbers dva (two), tri (three), and četiri (four), nouns must take the genitive singular form. This rule overrides the sentence's overall case, which is why we do not use the accusative plural krumpire even though the potatoes are the direct object.

Why use the perfective verb oguli instead of the imperfective guli?

This is a specific instruction to complete a task. The prefix o- makes the verb perfective, meaning the focus is on the finished result (fully peeled potatoes). If you used guli, it would sound like a general command to spend some time peeling without necessarily finishing the job.

Is prvo an adjective or an adverb in this sentence?

It acts as an adverb here, meaning 'firstly' or 'first of all'. If it were an adjective modifying a noun (like 'the first potato'), it would change its ending to match the noun's gender, number, and case, such as prvi krumpir.

Should I include the pronoun ti (you) to make this command clearer or more polite?

No, adding the pronoun ti does not make it more polite; it adds contrastive emphasis. Saying Prvo ti oguli tri krumpira sounds like 'First, you peel three potatoes (and someone else will do the rest)'. For standard instructions, always drop the pronoun.