Kad jedrilica podigne sidro, mali brod polako izlazi iz luke.

Because Kad jedrilica podigne sidro is a subordinate time clause introduced by kad.

In Croatian, when a subordinate clause comes before the main clause, it is normally separated by a comma:

• Kad jedrilica podigne sidro, mali brod polako izlazi iz luke.

If you reverse the order, you still usually keep the comma:

• Mali brod polako izlazi iz luke kad jedrilica podigne sidro.

So the comma is doing the same basic job as in English with an initial when clause.

They both mean when.

The difference is mostly style:

• kad = more common in everyday speech and writing
• kada = a bit more formal, fuller, or more emphatic

So this sentence could also be:

• Kada jedrilica podigne sidro, mali brod polako izlazi iz luke.

That would still be correct, just slightly less conversational.

Because podigne is a perfective present form, and Croatian often uses a perfective present after kad to mean when/once an action is completed.

Here the idea is:

• first the sailboat raises anchor
• then the other boat leaves

So podigne presents the anchor-raising as a completed trigger.

Very roughly:

• kad ... podigne ... = when/once it raises
• kad ... podiže ... would suggest when it is raising or whenever it raises/is in the process of raising, which is a different nuance

So podigne is the natural choice if the action is seen as a completed event that leads to the next one.

Podigne is:

• 3rd person singular
• present tense form
• from a perfective verb such as podići / podignuti

It agrees with jedrilica, which is singular:

• jedrilica podigne = the sailboat raises

Even though English may sometimes translate this with future meaning in context, Croatian still uses this present form after kad.

Izlazi is from the imperfective verb izlaziti.

The imperfective here gives the sense of an action in progress or unfolding gradually, which fits well with polako:

• polako izlazi = slowly comes out / slowly is leaving

If you used a perfective verb such as izađe, the focus would shift more toward the completed result:

• izađe = goes out / exits completely

So:

• izlazi = emphasizes the process
• izađe = emphasizes the completion

With polako, the imperfective sounds especially natural.

Most naturally, it sounds like a general sequence or a vivid present-time description.

Why? Because the main clause uses izlazi, a present imperfective form. That often suggests:

• a habitual situation: whenever the sailboat raises anchor, the small boat slowly leaves the harbor
• or a descriptive/narrative present

If you wanted to make a single future event more explicit, Croatian would often use a future form in the main clause, for example:

• Kad jedrilica podigne sidro, mali brod će polako izaći iz luke.

So the given sentence is not wrong, but it sounds more like a regular sequence or a descriptive present than a plain one-time future statement.

Sidro is in the accusative singular, because it is the direct object of podigne.

The verb is acting on the anchor:

• podigne što? → sidro

For this neuter noun, the nominative and accusative singular look the same:

• nominative: sidro
• accusative: sidro

So even though the form does not change, the case is accusative here.

Because the preposition iz requires the genitive case.

The noun is luka in the nominative singular, but after iz it becomes genitive singular:

• nominative: luka
• genitive: luke

So:

• iz luke = out of the harbor / from the harbor

This is a very important pattern in Croatian:

• iz + genitive

It may look repetitive to an English speaker, but it is normal Croatian.

• izlazi is the verb comes out / goes out / exits
• iz luke tells you out of what place: out of the harbor

So the structure is basically:

• izlazi iz luke = goes out of the harbor

English does something similar:

• goes out of the harbor

So even though izlazi contains the idea of outward movement, Croatian still normally adds iz + genitive to name the place being exited.

Yes. Croatian word order is much more flexible than English word order.

This sentence could be rearranged, for example:

• Mali brod polako izlazi iz luke kad jedrilica podigne sidro.

The meaning stays basically the same, but the emphasis changes.

The original order:

• Kad jedrilica podigne sidro, ...

puts the time condition first. It sets the scene before telling you what happens next.

Also, polako before the verb is natural and highlights the slow movement:

• polako izlazi

Because Croatian does not have articles like English a and the.

So nouns such as:

• jedrilica
• sidro
• mali brod
• luka

can mean a or the, depending on context.

Croatian expresses definiteness through context, word order, adjectives, demonstratives, and general situation, not through articles.

So mali brod can mean:

• a small boat
• the small boat

depending on the context you are already in.