Vrabac kljuca zrno kukuruza.

Here are the base forms you would look up:

• vrabac — masculine noun
• kljucati — verb; here it appears as kljuca
• zrno — neuter noun
• kukuruz — masculine noun; here it appears as kukuruza

So the sentence contains one subject noun, one verb, one direct object, and then a noun in the genitive that depends on zrno.

Vrabac is in the nominative singular, which is the normal case for the subject.

Also, the verb kljuca is 3rd person singular present, so it matches a singular subject like vrabac.

In a neutral Croatian sentence, the subject often comes before the verb, so Vrabac kljuca... is a very natural way to say it.

Because zrno is a neuter singular noun, and in Croatian the nominative singular and accusative singular are often the same for neuter nouns.

So here:

• zrno = accusative singular
• its role is the direct object of kljuca

English speakers often expect the object to have a clearly different form, but Croatian does not always show that visibly on every noun.

Because kukuruza is genitive singular.

After a noun like zrno, Croatian often uses the genitive to show what the grain is of:

• zrno kukuruza = a grain/kernel of corn

This is not really possession. It is more like a part-whole or type/content relationship.

Compare similar patterns:

• čaša vode = a glass of water
• komad kruha = a piece of bread
• zrno pijeska = a grain of sand

Kljuca is:

• present tense
• 3rd person singular
• from the verb kljucati

So it means something like pecks or is pecking, depending on context.

Croatian does not have a separate form exactly like the English present progressive, so the same present-tense form can cover both a general action and an action happening now.

Because Croatian has no articles.

So vrabac can mean:

• a sparrow
• the sparrow

And zrno can mean:

• a grain
• the grain

Context tells you which meaning is intended. This is one of the biggest differences from English.

Yes, but the meaning changes a little.

• Vrabac kljuca zrno kukuruza. focuses on one grain/kernel
• Vrabac kljuca kukuruz. sounds more general, like the sparrow is pecking at corn as a substance or food

So zrno kukuruza is more specific.

Yes. Croatian word order is flexible, but different orders change the focus or emphasis.

• Vrabac kljuca zrno kukuruza. — neutral
• Zrno kukuruza kljuca vrabac. — emphasizes what is being pecked
• Kljuca zrno kukuruza vrabac. — more marked or stylistic

For a normal, neutral statement, the original order is the best choice.

Yes, if the subject is already clear from context.

Croatian is a pro-drop language, which means the subject pronoun or noun can often be omitted:

• Kljuca zrno kukuruza.

That would mean He/She/It is pecking a grain of corn, with the subject understood from the situation.

In your original sentence, vrabac is included because the speaker wants to name the subject explicitly.

A rough English-friendly guide is:

• vrabac ≈ VRA-bats
• kljuca ≈ KLYOO-tsa
• zrno ≈ ZR-no
• kukuruza ≈ koo-koo-ROO-za

Two especially useful pronunciation notes:

• c is always pronounced like ts in cats
• lj is a single sound, similar to the lli sound some English speakers use in million

Also, in zrno, the r is pronounced clearly and can almost act like a vowel, which may feel unusual at first to English speakers.