U veljači moja prijateljica obično planira kratko putovanje, jer joj veljača uvijek prođe prebrzo.

Because after u meaning in (for time expressions like months), Croatian uses the locative case.

• dictionary form: veljača = February
• locative singular: veljači
• so: u veljači = in February

This is the normal pattern for months:

• u siječnju = in January
• u veljači = in February
• u ožujku = in March

So u veljači is simply the correct case form after u in this meaning.

In Croatian, months are not capitalized unless they begin a sentence.

So:

• veljača = February
• siječanj = January
• travanj = April

This is different from English, where month names are always capitalized.

That is why you see:

• U veljači...
• ...jer joj veljača uvijek...

The first V is capitalized only because it comes at the start of the sentence.

Moja prijateljica is in the nominative case because it is the subject of the main clause.

• moja = my
• prijateljica = female friend
• together: moja prijateljica = my friend

She is the one doing the action of planira (plans), so nominative is used.

Because putovanje is a neuter noun, and the adjective must agree with it in gender, number, and case.

• putovanje = trip / journey, neuter singular
• adjective must therefore also be neuter singular
• kratko = short (neuter singular form)

So:

• kratko putovanje = a short trip

Compare:

• kratak plan = a short plan (masculine)
• kratka priča = a short story (feminine)
• kratko putovanje = a short trip (neuter)

Here it is also the direct object of planira, but for this neuter noun the accusative singular looks the same as the nominative singular.

Yes, Croatian word order is fairly flexible, and obično (usually) can often move.

In this sentence:

• moja prijateljica obično planira...

means my friend usually plans...

This placement is very natural and neutral.

You could also hear things like:

• U veljači obično moja prijateljica planira kratko putovanje
• Moja prijateljica planira kratko putovanje obično u veljači

But those versions can sound more marked, more focused, or less natural depending on context.

So the given position is a very standard one: the adverb comes before the main verb.

Because jer introduces a subordinate clause meaning because.

The sentence has two parts:

1. U veljači moja prijateljica obično planira kratko putovanje
2. jer joj veljača uvijek prođe prebrzo

Croatian normally separates these with a comma when jer introduces the reason.

So this is standard punctuation.

Jer means because.

In many situations, it is close in meaning to zato što, which also means because.

So these are similar:

• ..., jer joj veljača uvijek prođe prebrzo.
• ..., zato što joj veljača uvijek prođe prebrzo.

However, jer is often a little simpler and very common in everyday language.

So in this sentence, jer introduces the reason why she usually plans a short trip in February.

Joj is the dative singular form of the pronoun ona (she / her).

Here it means roughly to her.

So:

• joj veljača uvijek prođe prebrzo

literally feels like:

• February always passes too quickly to her

In more natural English: February always goes by too quickly for her.

This dative pronoun is very common in Croatian for the person affected by an experience:

• Hladno joj je. = She is cold. / It is cold to her.
• Dan mu je brzo prošao. = The day passed quickly for him.
• Vrijeme nam sporo prolazi. = Time passes slowly for us.

So joj marks the person who experiences February as passing too fast.

Because joj is a clitic, and Croatian clitics tend to appear in a special early position in the clause, often called the second position.

In practice, this means short unstressed words like mi, ti, mu, joj, se, ga, je, smo often come very early in the sentence or clause.

So:

• jer joj veljača uvijek prođe prebrzo

is natural because joj appears right after jer.

This word order is much more natural than putting joj later.

Croatian often repeats the noun when that sounds clearer or more natural.

Here:

• ...jer joj veljača uvijek prođe prebrzo

sounds very natural.

You could theoretically imagine a version with a pronoun, but repeating veljača works well because it keeps the sentence clear and smooth. It also avoids any moment of doubt about what the pronoun refers to.

Croatian is often quite comfortable with repeating nouns where English might prefer a pronoun.

Prođe is from the verb proći, which usually means to pass, to go by, to elapse, or to be over depending on context.

In this sentence:

• veljača uvijek prođe prebrzo

means:

• February always goes by too quickly
• or February always passes too quickly

This is a very common use of proći for time periods:

• Dan je brzo prošao. = The day went by quickly.
• Ljeto je prebrzo prošlo. = The summer passed too quickly.

This is a very useful feature of Croatian.

Prođe is the present tense form of a perfective verb (proći). Perfective verbs usually focus on a completed whole event.

In English, learners often expect an imperfective verb for repeated actions, but Croatian can use a perfective present in general truths or habitual statements when each event is seen as a complete whole.

So:

• veljača uvijek prođe prebrzo

means that each February, taken as a whole, ends up passing too quickly.

This is slightly different in feel from an imperfective form like prolazi:

• veljača uvijek prolazi prebrzo

This is also understandable and possible, but prođe strongly presents February as a complete period that is over before she knows it.

So the perfective form is very natural here.

Prebrzo means too quickly or too fast.

It is formed from:

• brzo = quickly / fast
• pre- = too, overly, excessively

So:

• brzo = quickly
• prebrzo = too quickly

This prefix is very common:

• previše = too much
• predugo = too long
• preskupo = too expensive
• prehladno = too cold

So prođe prebrzo = passes too quickly.

Yes: the sentence is natural, but Croatian word order is more flexible than English.

The structure is roughly:

• U veljači = time expression
• moja prijateljica = subject
• obično planira = adverb + verb
• kratko putovanje = object
• jer joj veljača uvijek prođe prebrzo = reason clause

A very literal breakdown would be:

• In February my friend usually plans a short trip, because to her February always passes too quickly.

What may feel unusual to an English speaker is mainly:

• the month in the locative: u veljači
• the clitic joj appearing early
• the use of prođe to describe a time period passing

But as a Croatian sentence, the word order is quite normal and natural.