Petnaestog srpnja krećemo rano, jer trajekt za otok polazi u osam.

In Croatian, dates are usually expressed with the ordinal number and the month in the genitive case:

• petnaesti = fifteenth
• srpanj = July
• srpnja = of July

So Petnaestog srpnja means on the fifteenth of July.

The form petnaestog is also genitive, because this date expression is historically understood as something like petnaestog dana srpnja = the fifteenth day of July. In modern Croatian, dana is usually left out, but the case remains.

Yes, literally it looks like a genitive expression, but in Croatian this is a normal way to say a date without a preposition.

English says:

• on the fifteenth of July

Croatian often simply says:

• Petnaestog srpnja

So you should learn it as a standard date expression, not translate it word-for-word. Croatian does not need a separate word corresponding to English on here.

Srpanj is the Croatian name for July. Croatian month names are Slavic in origin, unlike the Latin-based names used in English.

Here are the relevant forms:

• nominative: srpanj
• genitive: srpnja

Since dates use the genitive for the month, you get petnaestog srpnja.

Croatian often uses the present tense for a planned or scheduled future event, just like English can do in sentences such as:

• We leave early tomorrow.
• The ferry departs at eight.

So:

• krećemo rano = we’re leaving early / we leave early
• trajekt polazi u osam = the ferry leaves at eight

This is especially common when talking about plans, timetables, and arrangements.

It depends on context, but here krećemo means something like:

• we leave
• we set off
• we start out

Because the sentence is about getting somewhere in time for a ferry, we leave early is the most natural English translation.

The basic verb is related to movement or starting a journey.

Rano is an adverb, meaning early.

It describes how/when we leave:

• krećemo rano = we’re leaving early

Compare:

• rani autobus = an early bus / a bus in the early part of the day
  here rani is an adjective
• krećemo rano = we leave early
  here rano is an adverb

Because jer means because, and it introduces a subordinate clause.

So the sentence has two parts:

• Petnaestog srpnja krećemo rano
• jer trajekt za otok polazi u osam

Croatian normally places a comma before jer, just as English often separates the because clause in longer sentences.

Yes, you could also say:

• Petnaestog srpnja krećemo rano, zato što trajekt za otok polazi u osam.

Both jer and zato što can mean because.

Very roughly:

• jer is short, common, and straightforward
• zato što is also very common, sometimes a little fuller or more emphatic

In this sentence, jer sounds completely natural.

Here za means to or for, in the sense of destination:

• trajekt za otok = the ferry to the island

After za, Croatian uses the accusative case, and otok is the accusative form here.

Other examples:

• autobus za Zagreb = the bus to Zagreb
• vlak za Split = the train to Split

So za is a very common way to express destination in this kind of phrase.

Because after za meaning destination, Croatian uses the accusative case.

For the noun otok:

• nominative: otok
• accusative: otok

Since otok is an inanimate masculine noun, the accusative singular is the same as the nominative singular. So the form does not change, even though the case has changed.

Polazi means departs / leaves / sets off. It is often used for transport and scheduled departures:

• Vlak polazi u šest. = The train leaves at six.
• Trajekt polazi u osam. = The ferry departs at eight.

Ide is much broader and often just means goes:

• Autobus ide do centra. = The bus goes to the centre.

In this sentence, polazi is better because it focuses on the departure time.

Yes, literally u often means in, but with clock time Croatian uses u where English uses at.

So:

• u osam = at eight
• u pet = at five
• u podne = at noon

This is just the normal Croatian pattern for specific times.

Yes. Croatian often omits sati when the meaning is obvious.

So these mean the same thing:

• polazi u osam
• polazi u osam sati

The shorter version is very common in everyday speech.

Because Croatian has no articles at all. There is no direct equivalent of a/an/the.

So:

• trajekt can mean a ferry or the ferry
• otok can mean an island or the island

The context tells you which meaning is intended. In this sentence, trajekt za otok is naturally understood as the ferry to the island.

Yes, Croatian word order is more flexible than English word order, although some versions sound more natural than others.

The original sentence:

• Petnaestog srpnja krećemo rano, jer trajekt za otok polazi u osam.

This is neutral and natural.

You could also move some elements for emphasis, for example:

• Krećemo rano petnaestog srpnja, jer trajekt za otok polazi u osam.
• Jer trajekt za otok polazi u osam, petnaestog srpnja krećemo rano.

But the original order is the clearest and most standard for a learner.

Very literally, it is something like:

• Fifteenth-of July we-set-off early, because ferry to island departs at eight.

That is not natural English, of course, but it helps show the structure:

• Petnaestog srpnja = on the fifteenth of July
• krećemo rano = we leave early
• jer = because
• trajekt za otok = the ferry to the island
• polazi u osam = departs at eight